Product Guides
Tap Drill Size Chart: Metric & Imperial Thread Sizes
Tap drill size is the diameter of the pilot hole drilled before threading. The rule: drill diameter = thread outer diameter − thread pitch. For M8 × 1.25 coarse, the tap drill is 6.8mm. For 1/4" BSP, it's 11.8mm. The compact reference below covers the most-used metric coarse sizes; full metric fine, BSP, UNC and UNF charts are further down. (If you are looking for the Sutton Tap & Drill Chart PDF, click here.) Quick answer — most common sizes Metric coarse: M3 = 2.5mm · M4 = 3.3mm · M5 = 4.2mm · M6 = 5.0mm · M8 = 6.8mm · M10 = 8.5mm · M12 = 10.2mm · M14 = 12.0mm · M16 = 14.0mm · M20 = 17.5mm BSP: 1/8" = 8.8mm · 1/4" = 11.8mm · 3/8" = 15.0mm · 1/2" = 18.6mm UNC: 1/4"-20 = 5.1mm · 5/16"-18 = 6.9mm · 3/8"-16 = 7.9mm · 1/2"-13 = 10.7mm Formula: tap drill (mm) = thread OD − pitch. Full charts below. For more engineering reference charts and selection tables, see our Engineering Reference Charts hub — covering fasteners, bearings, lubrication, measuring, welding and Australian standards. Tap & Drill Bit Selector — Most-Asked Metric Sizes This page is a working selector tool — not just a reference. Use it to get the right tap and drill into your hand in one click. The 10 most-asked metric thread sizes at AIMS are below. For less common sizes, scroll to the full charts further down (use the jump-nav below). How to use: 1. Find your thread size 2. See the matching tap drill diameter 3. Click Buy Tap or Buy Drill — sized to match M3 Tap drill 2.5 mm Buy tap → Buy drill → M4 Tap drill 3.3 mm Buy tap → Buy drill → M5 Tap drill 4.2 mm Buy tap → Buy drill → M6 Tap drill 5.0 mm Buy tap → Buy drill → M8 Tap drill 6.8 mm Buy tap → Buy drill → M10 Tap drill 8.5 mm Buy tap → Buy drill → M12 Tap drill 10.2 mm Buy tap → Buy drill → M14 Tap drill 12.0 mm Buy tap → Buy drill → M16 Tap drill 14.0 mm Buy tap → Buy drill → M20 Tap drill 17.5 mm Buy tap → Buy drill → Default recommendation: Sutton Spiral Point HSS taps and Sutton D101 Silver Bullet HSS jobber drills — the workshop standard for mild steel. AIMS also stocks Bordo and P&N as alternates. For stainless steel or hardened steel, switch to cobalt drill bits + cobalt taps (see the "By Material" section below). Need help? Call us on (02) 9773 0122. Jump to: Metric Coarse Metric Fine BSP UNC UNF By Material Related Selectors Tap Drill Size Chart — Metric Coarse Quick Reference The most frequently used metric coarse thread sizes and their tap drill diameters: Thread Tap Drill (mm) Thread Tap Drill (mm) M3 2.5 M12 10.2 M4 3.3 M14 12.0 M5 4.2 M16 14.0 M6 5.0 M18 15.5 M8 6.8 M20 17.5 M10 8.5 M24 21.0 How to Use This Chart Tap drill size refers to the diameter of the hole you drill before running a tap through it. The hole must be smaller than the thread's outer diameter, leaving enough material for the tap to cut the thread profile. Too large and the thread is shallow and weak. Too small and you risk breaking the tap. As a general rule, tap drill size = thread outer diameter − thread pitch. This gives approximately 75% thread engagement, which is standard for most applications. For softer materials or where tap breakage is a concern, go slightly larger. For maximum thread strength in hard materials, go slightly smaller. By accurately matching the tap size to the drill size and choosing the right tap for the job, you can achieve optimal results in your thread cutting operations. Where a bolt or screw passes through a hole rather than being tapped into it, you need a clearance hole instead — sized so the fastener slides through freely rather than cutting a thread. The metric tables below include a Clearance Hole (mm) column for this, based on the ISO 273 normal (medium) fit for the diameters most commonly used in through-bolt assembly. A dash (—) marks sizes that fall outside the standard clearance-hole table — generally uncommon "second choice" diameters rarely used for through-bolt work. Metric Coarse Tap Drill Size Chart Metric coarse is the standard thread series for most bolts, screws and tapped holes in general engineering. Pitch is expressed in millimetres — a lower number means finer threads. These are the sizes you'll use for the vast majority of metric tapping work. Need help finding your size? Call AIMS on (02) 9773 0122 — the team can confirm stock and pick the right tap type for your job. Or browse all metric coarse taps and jobber drill bits. Thread Size Pitch (mm) Tap Drill (mm) Tap Drill (inch approx.) Clearance Hole (mm) M1 0.25 0.75 --- — M1.2 0.25 0.95 --- — M1.4 0.30 1.10 --- — M1.6 0.35 1.25 --- 1.8 M1.8 0.35 1.45 --- — M2 0.40 1.60 1/16" 2.4 M2.5 0.45 2.05 5/64" 2.9 M3 0.50 2.50 3/32" 3.4 M3.5 0.60 2.90 7/64" — M4 0.70 3.30 1/8" 4.5 M5 0.80 4.20 11/64" 5.5 M6 1.00 5.00 13/64" 6.6 M7 1.00 6.00 15/64" — M8 1.25 6.80 17/64" 9.0 M10 1.50 8.50 21/64" 11.0 M12 1.75 10.20 25/64" 13.5 M14 2.00 12.00 15/32" 15.5 M16 2.00 14.00 35/64" 17.5 M18 2.50 15.50 39/64" — M20 2.50 17.50 11/16" 22.0 M22 2.50 19.50 49/64" — M24 3.00 21.00 53/64" 26.0 M27 3.00 24.00 15/16" — M30 3.50 26.50 1-3/64" 33.0 M33 3.50 29.50 1-5/32" — M36 4.00 32.00 1-17/64" 39.0 M39 4.00 35.00 1-3/8" — M42 4.50 37.50 1-15/32" 45.0 M45 4.50 40.50 1-19/32" — M48 5.00 43.00 1-11/16" 52.0 Metric Fine Tap Drill Size Chart Metric fine threads are used where vibration resistance, fine adjustment, or higher tensile strength is required — common in automotive, aerospace, and precision engineering applications. Multiple pitches exist per diameter; confirm your pitch before selecting the drill. Need help finding your size? Call AIMS on (02) 9773 0122 — the team can confirm stock and pick the right tap type for your job. Or browse all metric fine taps and jobber drill bits. Thread Size Pitch (mm) Tap Drill (mm) Clearance Hole (mm) M1 × 0.2 0.20 0.80 — M1.2 × 0.2 0.20 1.00 — M1.4 × 0.2 0.20 1.20 — M1.6 × 0.2 0.20 1.40 1.8 M2 × 0.25 0.25 1.75 2.4 M2.5 × 0.35 0.35 2.15 2.9 M3 × 0.35 0.35 2.65 3.4 M3.5 × 0.35 0.35 3.15 — M4 × 0.5 0.50 3.50 4.5 M5 × 0.5 0.50 4.50 5.5 M6 × 0.75 0.75 5.25 6.6 M7 × 0.75 0.75 6.25 — M8 × 0.75 0.75 7.25 9.0 M8 × 1.0 1.00 7.00 9.0 M10 × 0.75 0.75 9.25 11.0 M10 × 1.0 1.00 9.00 11.0 M10 × 1.25 1.25 8.75 11.0 M12 × 1.0 1.00 11.00 13.5 M12 × 1.25 1.25 10.75 13.5 M12 × 1.5 1.50 10.50 13.5 M14 × 1.0 1.00 13.00 15.5 M14 × 1.25 1.25 12.75 15.5 M14 × 1.5 1.50 12.50 15.5 M16 × 1.0 1.00 15.00 17.5 M16 × 1.5 1.50 14.50 17.5 M18 × 1.5 1.50 16.50 — M18 × 2.0 2.00 16.00 — M20 × 1.5 1.50 18.50 22.0 M20 × 2.0 2.00 18.00 22.0 M22 × 1.5 1.50 20.50 — M22 × 2.0 2.00 20.00 — M24 × 1.5 1.50 22.50 26.0 M24 × 2.0 2.00 22.00 26.0 M27 × 2.0 2.00 25.00 — M30 × 1.5 1.50 28.50 33.0 M30 × 2.0 2.00 28.00 33.0 M33 × 2.0 2.00 31.00 — M36 × 1.5 1.50 34.50 39.0 M36 × 3.0 3.00 33.00 39.0 Tap & Drill Selection by Material The right tap drill diameter is one half of the job. The other half is choosing tap and drill geometry that match your workpiece material. This is where most beginner tappers come unstuck — wrong tap type for the material means broken taps, oversized threads, or torn surface finishes. Here's what the AIMS workshop crew reaches for, by material: Mild steel (most common workshop material) Drill: Sutton D101 Silver Bullet HSS jobber (bright finish, 118° point) for occasional tapping. Sutton D102 Blue Bullet HSS (steam-oxide finish) for production tapping — the steam oxide finish helps swarf release on steel. Tap: Sutton Spiral Point HSS taps (T1xx series) for through holes — fastest, swarf pushes ahead of the tap. For blind holes, switch to Spiral Flute (T2xx series) so swarf evacuates upward and out of the hole. Lubricant: Tap Magic Original or any general-purpose cutting fluid. Stainless steel (304, 316, 17-4 PH) Drill: Cobalt drill bit — Sutton D108 or D109 cobalt jobber. The 5% to 8% cobalt content lets the drill stay sharp through stainless's work-hardening tendency. HSS bright drills will glaze the hole surface, work-harden the steel, and snap on the next pass. Tap: Cobalt steel tap — Sutton Spiral Flute Premium HSS Cobalt or Sutton Premium HSS Tinite-coated. The cobalt grade survives the harder material; cheap HSS chrome taps will snap on the first thread. Lubricant: Tap Magic for Stainless Steel — specifically formulated. Don't skimp here. Aluminium, brass, copper, plastics Drill: Standard HSS Sutton D101 Silver Bullet is fine. Some users prefer a slightly higher helix angle drill for softer non-ferrous materials — Sutton's 130° or 140° point geometry options. Tap: Spiral Flute tap is excellent here — long stringy chips need to evacuate cleanly, and spiral flute pulls them up and out. Avoid spiral point in soft aluminium (it bunches chips inside the hole). Lubricant: Tap Magic Aluminium variant — formulated to prevent the gummy build-up that aluminium causes on standard cutting fluids. Cast iron (grey, ductile, malleable) Drill: HSS Sutton D102 Blue Bullet (steam-oxide). Cast iron is brittle and abrasive — the steam oxide finish helps prolong drill life. Tap: Straight Flute tap (T4xx series) or hand tap. Cast iron breaks into powder rather than chips, so spiral evacuation isn't needed — straight flute is more rigid and handles the abrasiveness better. Lubricant: Generally tapped DRY — no cutting fluid needed for cast iron because chips are powder, not strings. Hardened steel (above ~30 HRC) For anything above mild steel hardness — pre-hardened tool steels, heat-treated parts, hardfaced surfaces — call AIMS before you start drilling. Solid carbide drills + thread mills are the right answer here, not standard taps. Contact us or call (02) 9773 0122 — we'll save you broken taps and damaged workpieces. The AIMS workshop rule: The right cutting fluid is worth more than the right tap. Even a premium Sutton tap will fail prematurely if you're tapping stainless without proper lubrication. Tap Magic cutting fluid guide covers which formulation matches your material. BSP Tap Drill Size Chart (British Standard Pipe) BSP threads are used on pipe fittings, hydraulic connections, and pneumatic systems throughout Australia and the UK. Sizes refer to the nominal bore of the pipe — not the actual thread diameter, which is always larger. BSPP (parallel) and BSPT (taper) share the same thread form and the same tap drill size. Need help finding your size? Call AIMS on (02) 9773 0122 — the team can confirm stock and pick the right tap type for your job. Or browse all BSP taps and jobber drill bits. Nominal Size TPI Tap Drill (mm) Tap Drill (inch) 1/16" BSP 28 6.6 0.261" 1/8" BSP 28 8.8 0.347" 1/4" BSP 19 11.8 0.465" 3/8" BSP 19 15.0 0.590" 1/2" BSP 14 18.6 0.733" 3/4" BSP 14 24.3 0.956" 1" BSP 11 30.5 1.200" 1¼" BSP 11 39.2 1.544" 1½" BSP 11 45.1 1.776" 2" BSP 11 57.0 2.245" 2½" BSP 11 72.6 2.858" 3" BSP 11 87.8 3.457" The 1/4" BSP tap drill size (11.8mm) is one of the most commonly referenced in Australian trade and industrial work. If you're unsure whether your fitting is BSPP or BSPT, the tap drill size is the same for both — the distinction only matters when selecting the tap itself. UNC Tap Drill Size Chart (Unified National Coarse) UNC is the standard US coarse thread series. Common in imported machinery, agricultural equipment, and items manufactured to American standards. Identified by thread count in threads per inch (TPI). Need help finding your size? Call AIMS on (02) 9773 0122 — the team can confirm stock and pick the right tap type for your job. Or browse all UNC taps and jobber drill bits. Thread TPI Tap Drill (mm) Tap Drill (fractional inch) #4-40 40 2.4 3/32" #5-40 40 2.65 --- #6-32 32 2.8 7/64" #8-32 32 3.5 9/64" #10-24 24 3.9 5/32" 1/4"-20 20 5.1 13/64" 5/16"-18 18 6.9 17/64" 3/8"-16 16 7.9 5/16" 7/16"-14 14 9.4 3/8" 1/2"-13 13 10.7 27/64" 9/16"-12 12 12.3 31/64" 5/8"-11 11 13.5 17/32" 3/4"-10 10 16.7 21/32" 7/8"-9 9 19.4 49/64" 1"-8 8 22.2 7/8" 1-1/8"-7 7 25.4 1" 1-1/4"-7 7 28.6 1-1/8" 1-3/8"-6 6 31.0 1-7/32" 1-1/2"-6 6 34.1 1-11/32" UNF Tap Drill Size Chart (Unified National Fine) UNF has a finer pitch than UNC — more threads per inch, higher tensile strength, and better vibration resistance. Used in aerospace, precision equipment, and anywhere a finer thread is specified. When in doubt, check the thread count: more threads per inch means UNF. Need help finding your size? Call AIMS on (02) 9773 0122 — the team can confirm stock and pick the right tap type for your job. Or browse all UNF taps and jobber drill bits. Thread TPI Tap Drill (mm) Tap Drill (fractional inch) #4-48 48 2.3 3/32" #6-40 40 2.9 7/64" #8-36 36 3.5 9/64" #10-32 32 3.8 9/64" 1/4"-28 28 5.6 7/32" 5/16"-24 24 6.9 17/64" 3/8"-24 24 8.5 21/64" 7/16"-20 20 9.9 25/64" 1/2"-20 20 11.5 29/64" 9/16"-18 18 13.1 33/64" 5/8"-18 18 14.7 37/64" 3/4"-16 16 17.5 11/16" 7/8"-14 14 20.6 13/16" 1"-12 12 23.4 59/64" 1-1/8"-12 12 26.6 1-3/64" 1-1/4"-12 12 29.8 1-3/16" 1-3/8"-12 12 33.0 1-5/16" 1-1/2"-12 12 36.5 1-7/16" Related AIMS Selectors & Guides This page sits at the centre of AIMS's threading and drilling tool cluster. The related selectors and selection guides below go deeper on the choices around it: Tap & Die Selection Guide — what tap type for which material, hole type, and machine Drill Bit Selection Guide — HSS vs cobalt vs carbide, jobber vs stub vs reduced shank Cutting Speeds & Feeds Reference — Vc and feed rate by material and tool type Cutting Tool Materials — HSS, cobalt, carbide, PCBN, PCD compared Cutting Tool Coatings — TiN, TiAlN, AlCrN, when each matters Cutting Tool Troubleshooting — broken taps, walking drills, poor finish, oversize holes Metric / Imperial / Gauge Conversion Master Chart — full drill bit + thread size cross-reference Thread Standards: BSP vs NPT vs UNC — identify the thread system you're dealing with Or browse the full taps range + jobber drill bits + cobalt drill bits — Sutton primary stock, Bordo and P&N alternates, specialty brands available for next-day Australia-wide dispatch from our Milperra warehouse.Frequently Asked Questions What drill size for M3 tap? For M3 coarse thread (0.5mm pitch), use a 2.5mm tap drill. For M3 fine (0.35mm pitch), use a 2.65mm drill. Formula: 3 − 0.5 = 2.5mm. What drill size for M4 tap? For M4 coarse thread (0.7mm pitch), use a 3.3mm tap drill. For M4 fine (0.5mm pitch), use a 3.5mm drill. Formula: 4 − 0.7 = 3.3mm. What drill size for M5 tap? For M5 coarse thread (0.8mm pitch), use a 4.2mm tap drill. For M5 fine (0.5mm pitch), use a 4.5mm drill. Formula: 5 − 0.8 = 4.2mm. What drill size for M6 tap? For M6 coarse thread (1.0mm pitch), use a 5.0mm tap drill. For M6 fine (0.75mm pitch), use a 5.25mm drill. Formula: 6 − 1.0 = 5.0mm. What drill size for M8 tap? For M8 coarse thread (1.25mm pitch), use a 6.8mm tap drill. For M8 fine (1.0mm pitch), use a 7.0mm drill. Formula: 8 − 1.25 = 6.75mm, rounded to 6.8mm. What drill size for M10 tap? For M10 coarse thread (1.5mm pitch), use an 8.5mm tap drill. For M10 fine (1.25mm pitch), use 8.75mm; for M10 fine (1.0mm pitch), use 9.0mm. Formula: 10 − 1.5 = 8.5mm. What drill size for M12 tap? For M12 coarse thread (1.75mm pitch), use a 10.2mm tap drill. For M12 fine (1.5mm pitch), use 10.5mm; for M12 fine (1.25mm pitch), use 10.75mm. Formula: 12 − 1.75 = 10.25mm, rounded to 10.2mm. What drill size for M14 tap? For M14 coarse thread (2.0mm pitch), use a 12.0mm tap drill. For M14 fine (1.5mm pitch), use 12.5mm. Formula: 14 − 2.0 = 12.0mm. What drill size for M16 tap? For M16 coarse thread (2.0mm pitch), use a 14.0mm tap drill. For M16 fine (1.5mm pitch), use 14.5mm. Formula: 16 − 2.0 = 14.0mm. What drill size for M20 tap? For M20 coarse thread (2.5mm pitch), use a 17.5mm tap drill. For M20 fine (2.0mm pitch), use 18.0mm; for M20 fine (1.5mm pitch), use 18.5mm. Formula: 20 − 2.5 = 17.5mm. What drill size for 1/4 inch BSP tap? The recommended tap drill for 1/4 inch BSP (19 TPI) is 11.8mm, or 0.465 inches. This applies to both BSPP (parallel) and BSPT (taper) threads — the tap drill is the same. What drill size for 1/8 inch NPT tap? The recommended tap drill for 1/8 inch NPT (27 TPI) is 8.6mm, or 21/64 inch. NPT is a tapered thread used on American pipe fittings — distinct from BSP. What drill size for 1/4 inch UNC tap? For 1/4-20 UNC, use a 5.1mm tap drill (13/64 inch). UNC has 20 threads per inch and is the standard US coarse thread. How do I calculate tap drill size for metric threads? Tap drill size (mm) equals thread diameter minus thread pitch. Example: M10 × 1.5 = 10 − 1.5 = 8.5mm. This gives approximately 75% thread engagement, which is standard for most applications. What is a tap drill size? A tap drill size is the diameter of the hole you drill before cutting a thread with a tap. It must be smaller than the thread's outer diameter so the tap has material to cut the thread profile into. What is the difference between BSPP and BSPT? BSPP (British Standard Pipe Parallel) has straight threads and seals with an O-ring or washer. BSPT (British Standard Pipe Taper) has a tapered thread that seals as it tightens. Both share the same tap drill size for a given nominal size, but BSPT taps are designed to cut a taper. If a tap breaks during the threading process, see our guide on how to remove a broken tap — covering all six removal methods from tap extractors through to EDM. Ready to tap? Shop our full range of taps, dies & threading tools From metric hand taps to imperial die sets — AIMS Industrial stocks threading tools for every standard, ready to ship Australia-wide. Browse taps Talk to a specialist For the tap type that matches your hole and material, see our Tap Types Explained guide. For choosing the right cutting fluid for your material, see our Tap Magic cutting fluid guide. Need to identify a thread standard? Our Thread Standards Guide covers BSP, NPT, UNC, UNF, BSW and metric with identification tips. People Also Ask — Tap Drill Size Chart: Metric & Imperial Thread Sizes Q: What drill size do I use for an M10 tap? For M10 × 1.5 coarse thread, use an 8.5 mm pilot drill. For M10 × 1.25 fine thread, use a 8.75 mm drill (often rounded to 9.0 mm in practice). Using the correct pilot hole is critical — too small risks tap breakage; too large produces insufficient thread engagement and weak joints. Q: What is the difference between a taper tap and a plug tap? A taper tap has a long chamfer (8–10 threads) that guides it into the hole gradually — best for starting threads in blind or through holes. A plug tap (4–5 thread chamfer) picks up where the taper left off and is the most common general-purpose tap. A bottoming tap has just 1–2 threads of chamfer for cutting threads to the very bottom of a blind hole. Q: What is the difference between BSP and NPT threads? BSP (British Standard Pipe) uses a 55° thread angle and is the standard for most Australian, British, and European hydraulic and pneumatic fittings. NPT (National Pipe Taper) uses a 60° thread angle and is common on American equipment. The two are not interchangeable — mismatching causes leaks and can damage fittings even if they appear to thread together. Q: How do I know which tap size to buy? Match the tap to the bolt thread you need — an M6 × 1.0 tap cuts the thread for an M6 coarse bolt. Always pair taps with the correct pilot drill from a tap drill chart. For blind holes, buy a taper, plug, and bottoming tap set. For through holes, a plug tap alone is usually sufficient for most trade applications. For machining, see our machining range stocked across Australia. Looking for long drill bits? Our long drill bits range covers the common sizes and brands. Prefer an interactive lookup? Use the AIMS Tap & Drill Selector tool to get the right tap and drill by thread type, size and material.
Read moreProduct Guides
AIMS Tap & Die Selection Guide: Hand, Spiral Point, Spiral Flute & Forming
A practical guide to selecting the right tap and die for the thread you're cutting. Covers hand taps (taper, plug, bottoming), spiral point taps for through holes, spiral flute taps for blind holes, thread forming taps for ductile material, machine nut taps, die nuts, button dies and die holders. Includes thread system selection (metric, BSP, BSPT, NPT, UNC, UNF, BSW, BSF), tap drill sizing, brand selection across Sutton Tools and Bordo, and AS/NZS/ISO standards.
Read moreThread Identification Guide: BSP, NPT, Metric, UNC, Whitworth & ACME
Thread Identification Quick Reference Use this table as your first check when identifying an unknown thread. The thread angle column is the fastest field identification method — 55° is always British-heritage (BSP or BSW), 60° is always American or metric. Standard Origin Thread Angle Profile Form Common AU Usage Key Identifying Feature BSP (BSPP/BSPT) British 55° Rounded crests & roots (Whitworth form) Hydraulics, compressed air, plumbing in AU/UK/EU 55° angle; nominal size = pipe bore, not thread OD NPT American 60° Flat crests, rounded roots US-import equipment, oilfield, pneumatic gear 60° angle + 1:16 taper; often confused with BSPT UNC / UNF American 60° Symmetric parallel (flat crests, flat roots) Imperial fasteners, US machinery, aerospace (UNF) 60° + specified in TPI; UNC coarser, UNF finer Metric M-series International (ISO) 60° Symmetric parallel (flat crests, rounded roots) All new AU manufacturing and imported equipment 60° + pitch in mm (not TPI); e.g. M10 × 1.5 BSW British 55° Rounded crests & roots (Whitworth form) Pre-1970s AU/British machinery, classic vehicles 55° + coarser pitch than BSF; shared TPI with UNC at some sizes BSF British 55° Rounded crests & roots (Whitworth form) Pre-1970s British vehicles, aviation legacy hardware 55° + finer pitch than BSW at same nominal size ACME American 29° Trapezoidal (flat top, flat root, angled flanks) Lead screws, vices, lathes, linear actuators Wide flat-topped thread visible to eye; 29° flanks Tr (Trapezoidal metric) International (ISO) 30° Trapezoidal (metric sizing) European CNC equipment, metric lead screws Wide flat-topped thread; 30° flanks; metric diameter notation If you've ever tried to thread a BSP fitting into an NPT port and felt it cross-thread, or wondered why an "imperial" thread won't bite into a metric hole — you've hit the wall every Australian tradie eventually meets: thread standards are not interchangeable. This guide breaks down the five major thread standards you'll encounter in Australian workshops, mines, factories and farms, and shows you how to identify each one before you ruin a fitting or a thread. Bookmark our Engineering Reference Charts hub for related sizing tables, conversion charts and Australian standard references across 9 topic clusters. Thread Standards — Quick Reference Standard Full Name Thread Angle Form Where You'll Find It BSP British Standard Pipe 55° Parallel (BSPP) or Tapered (BSPT) Plumbing, hydraulics, compressed air in AU/UK/EU NPT National Pipe Taper 60° Tapered (1:16) US-import equipment, oilfield, some pneumatic gear UNC Unified National Coarse 60° Symmetric parallel General-purpose imperial fasteners, US/AU/UK UNF Unified National Fine 60° Symmetric parallel Precision fasteners, automotive, vibration applications BSW British Standard Whitworth 55° Symmetric parallel Legacy Australian/British machinery, classic vehicles Key rule: the thread angle is the dead giveaway. Anything 55° is British heritage (BSP, BSW). Anything 60° is American or metric (NPT, UNC, UNF). Metric M-series is also 60°. For tap drill sizes for each standard, see our Tap Drill Size Chart (Metric & Imperial). For metric vs imperial fastener cross-references, see our Metric vs Imperial Fastener Guide. Why Thread Standards Matter Force a BSP male into an NPT female and you'll get a few turns of "almost right" before it binds, strips or cracks the casting. Force an NPT male into a BSP port and you'll either leak under pressure or split the receiving fitting. The cost ranges from a $5 replacement fitting to a multi-thousand-dollar pump housing — depending on what you've just destroyed. The problem is that thread standards share nominal sizes but use entirely different geometries: Different thread angle — 55° vs 60° means the peaks and valleys don't mesh Different pitch (TPI) at the same nominal size — even where the angle matches, the pitch may not Different sealing geometry — tapered threads seal by metal-to-metal interference, parallel threads need a separate seal Different nominal-size convention — BSP sizes refer to nominal bore, NPT sizes refer to a related but different reference An "M8" bolt and a "5/16" bolt look similar in your hand. They are not interchangeable. The same applies to thread standards in pipes and fittings. BSP — British Standard Pipe BSP (British Standard Pipe) is the dominant pipe thread standard across Australia, the UK, Europe, India, South Africa and most former British Commonwealth countries. If you're working with plumbing fittings, hydraulic fittings, compressed-air fittings, or pneumatic equipment in Australia, the default is BSP unless explicitly stated otherwise. BSP uses a 55° Whitworth thread form with rounded crests and roots. There are two BSP variants you need to know. BSPP — British Standard Pipe Parallel BSPP threads run straight along the length of the pipe (parallel — not tapered). The thread itself does not provide a seal; you need a separate sealing method, typically: Bonded seal washer (Dowty seal) — rubber-bonded steel washer compressed under the fitting head O-ring seated in a port face Flat washer with sealant BSPP is governed by ISO 228 (adopted in Australia as AS ISO 228). You'll see it on hydraulic adapters, compressed-air fittings, and most pneumatic gear in Australian workshops. BSPT — British Standard Pipe Taper BSPT threads are cut on a 1:16 taper (the same taper as NPT, which causes endless confusion — see below). The taper means the thread itself creates the seal as the male fitting wedges into the female port. PTFE tape or thread sealant is wound onto the male thread to fill the small voids and prevent capillary leaks, but the metal-to-metal taper does most of the work. BSPT is governed by ISO 7-1 (adopted as AS ISO 7). You'll see it on iron and brass pipe fittings, particularly water and gas plumbing. Australian BSP Size Reference Nominal Size TPI Pitch (mm) Male OD (mm) Tap Drill (mm) 1/8" 28 0.907 9.728 8.8 1/4" 19 1.337 13.157 11.8 3/8" 19 1.337 16.662 15.25 1/2" 14 1.814 20.955 19.0 3/4" 14 1.814 26.441 24.5 1" 11 2.309 33.249 30.75 1-1/4" 11 2.309 41.910 39.5 1-1/2" 11 2.309 47.803 45.25 2" 11 2.309 59.614 57.0 Critical detail: the BSP "size" is the nominal bore of the pipe it was originally designed for — NOT the actual diameter of the thread. A 1/2" BSP male thread has an outside diameter of approximately 21mm, not 12.7mm. This trips up tradies new to imperial pipe threading every single day. The tap drill column applies to BOTH BSPP and BSPT — the receiving hole is the same size; only the tap profile differs. For BSP fittings in stock, see our Brass Fittings, Iron Pipe Fittings, and Pipe Fittings collections. NPT — National Pipe Taper NPT (National Pipe Taper) is the American pipe thread standard. It's governed by ANSI/ASME B1.20.1. In Australia you'll encounter NPT mostly on imported equipment — particularly air compressors, hydraulic pumps, oilfield gear, and some American-made pneumatic tools. NPT uses a 60° symmetrical thread form with sharp crests and roots — completely different geometry from BSP's 55° rounded thread. Both NPT and BSPT use a 1:16 taper (3/4 inch per foot), which means the OD reduces as you move along the thread. That shared taper is the source of most BSP/NPT confusion — they look interchangeable, they bind for a few turns, then the angle mismatch ruins everything. NPT is ALWAYS tapered. There is also a parallel American thread (NPSF / NPSH / NPSM) but it is much less common — when someone says "NPT" they mean the tapered version. NPT Size Reference Nominal Size TPI Pitch (mm) Male OD at gauge plane (mm) Tap Drill (mm) 1/8" 27 0.941 10.272 8.6 1/4" 18 1.411 13.616 11.1 3/8" 18 1.411 17.055 14.5 1/2" 14 1.814 21.223 17.75 3/4" 14 1.814 26.568 23.25 1" 11.5 2.209 33.228 29.5 1-1/4" 11.5 2.209 41.985 38.0 1-1/2" 11.5 2.209 48.054 44.0 2" 11.5 2.209 60.092 55.5 Compare the BSP and NPT tables above and you'll see why they don't mix — at the same nominal size, the OD, TPI and tap drill are all subtly different. They're close enough to start threading. Close enough to fool a hurried tradie. Not close enough to seal. UNC vs UNF — Unified National Coarse vs Fine UNC and UNF are American imperial fastener thread standards (not pipe). Both use a 60° symmetric thread form, the same as metric M-series. The difference is purely the pitch — UNC has fewer threads per inch (coarse), UNF has more threads per inch (fine). Governed by ANSI/ASME B1.1. Common in Australia on: American-made vehicles (especially older Ford, Chev, Dodge — anything pre-metric conversion) Industrial machinery imported from the US Aerospace and marine applications (UNF dominates here) Some Australian-made gear that originally used Whitworth and converted to UN-series rather than metric UNC and UNF Sizes — Side by Side Nominal Size UNC TPI UNF TPI UNC Tap Drill (mm) UNF Tap Drill (mm) #6 32 40 2.85 2.95 #8 32 36 3.5 3.5 #10 24 32 3.9 4.1 1/4" 20 28 5.1 5.5 5/16" 18 24 6.5 6.9 3/8" 16 24 7.9 8.5 7/16" 14 20 9.4 9.9 1/2" 13 20 10.8 11.5 9/16" 12 18 12.2 13.0 5/8" 11 18 13.5 14.5 3/4" 10 16 16.5 17.5 7/8" 9 14 19.5 20.4 1" 8 12 22.25 23.25 When to choose UNC: general engineering, structural fastening, applications where you want fast assembly with hand tools. When to choose UNF: precision applications, thin-walled materials (more thread engagement per length), vibration-prone joints (the finer pitch resists self-loosening better), aerospace, motorsport. You cannot mix UNC and UNF at the same nominal size — a 1/2"-13 UNC bolt will not thread into a 1/2"-20 UNF nut, even though both are "imperial 1/2 inch". The pitch difference is the showstopper. BSW — British Standard Whitworth BSW (British Standard Whitworth) is the original imperial fastener thread invented by Joseph Whitworth in 1841. It uses the same 55° rounded thread form as BSP, but on solid fastener stock (not pipe). Defined by BS 84. You'll encounter BSW on: Pre-1970s Australian-made machinery — particularly Holdens, agricultural gear, and industrial plant Older British vehicles (Land Rover, BMC, Leyland, MG, etc.) Vintage tools and woodworking equipment Some legacy mining and railway equipment in Australia BSW is technically obsolete for new manufacturing — Australian industry transitioned to metric (and partly to UN-series) through the 1970s — but the legacy installed base is enormous. If you maintain old equipment in Australia, you'll meet BSW. Common BSW Sizes Nominal Size BSW TPI Pitch (mm) Tap Drill (mm) 1/8" 40 0.635 2.6 3/16" 24 1.058 3.7 1/4" 20 1.270 5.1 5/16" 18 1.411 6.5 3/8" 16 1.588 7.9 1/2" 12 2.117 10.5 5/8" 11 2.309 13.5 3/4" 10 2.540 16.5 1" 8 3.175 22.25 BSW vs UNC trap: 1/4"-20 BSW and 1/4"-20 UNC both have 20 threads per inch at 1/4" nominal — but the thread angle is different (55° vs 60°), so they don't mesh cleanly. Forcing them will work for a few turns, then bind or cross-thread. Metric M-Series Threads The metric M-series is the default thread standard for all new Australian manufacturing and most imported equipment. If the machinery was built after 1970, the fasteners are almost certainly metric unless it originates from the USA or is specifically identified as imperial. Metric threads use a 60° symmetric thread form (same angle as UNC/UNF) with flat crests and rounded roots. They are specified by nominal diameter in millimetres, followed by pitch in millimetres: M10 × 1.5 means 10mm nominal diameter, 1.5mm pitch (distance between thread crests). The governing standards are: ISO 68-1:2023 — general metric screw thread profile (the fundamental standard, now in its 2nd edition) ISO 261:1998 — metric screw thread general purpose sizes (the selection standard for preferred M-series sizes) ISO 262:1998 — selected metric screw thread sizes for screws, bolts and nuts AS 1275-1985 (reconfirmed 2017) — Australian adoption of the metric thread standard Metric Coarse (Preferred) Pitch — M3 to M30 Metric coarse pitch is the default — if a size is not marked as "fine" (F), assume coarse. Coarse pitch is faster to assemble, more tolerant of debris, and the correct choice for most general engineering applications. Size Coarse Pitch (mm) Tap Drill (mm) Minor Dia (mm) Common Application M3 0.5 2.5 2.459 Electronics, small instruments M4 0.7 3.3 3.242 Light machinery, switchgear M5 0.8 4.2 4.134 General engineering M6 1.0 5.0 4.917 Most common small fastener in AU workshops M8 1.25 6.8 6.647 Structural, automotive, machinery M10 1.5 8.5 8.376 Most common medium fastener M12 1.75 10.2 10.106 Structural steel, flanges M14 2.0 12.0 11.835 Automotive (cylinder head bolts) M16 2.0 14.0 13.835 Heavy structural, machinery bases M20 2.5 17.5 17.294 Foundation bolts, large structural connections M24 3.0 21.0 20.752 Large machinery, bridge structural M30 3.5 26.5 26.211 Heavy plant, foundation anchors Tap drill formula: Tap Drill = Nominal Diameter − Pitch (e.g. M10 × 1.5: tap drill = 10 − 1.5 = 8.5mm). This formula gives you 100% thread depth — in practice, 75% thread depth (drill slightly larger) is often preferred for easier tapping without significant strength loss. Metric Fine Pitch Metric fine pitch threads have smaller pitch at the same nominal diameter. For example, M10 × 1.25 (fine) vs M10 × 1.5 (coarse). Use metric fine where: Thin-walled components need maximum thread engagement per unit length Vibration resistance is required (finer pitch resists self-loosening) Precision adjustment is needed (e.g. bearing pre-load nuts, lock nuts on bearing housings) High-strength fasteners in automotive or motorsport applications Common metric fine sizes you'll encounter in Australian workshops: M8 × 1.0, M10 × 1.25, M12 × 1.25, M14 × 1.5, M16 × 1.5, M20 × 1.5. Metric vs Imperial — Quick Identification When you have an unknown fastener and need to determine metric or imperial quickly: Measure the pitch with a thread pitch gauge. If the pitch is a nice round millimetre number (1.0, 1.25, 1.5, 1.75, 2.0mm), it's metric. If it matches a TPI value (e.g. 20, 18, 16, 13 threads per inch), it's imperial. Measure the OD. Metric ODs are whole millimetre numbers (M8 OD = 8.0mm, M10 OD = 10.0mm, M12 OD = 12.0mm). Imperial ODs convert awkwardly (1/2" = 12.7mm, 5/8" = 15.875mm). Check the head markings. Metric grade marks are numbers (8.8, 10.9, 12.9). Imperial grade marks are lines (SAE Grade 5 = 3 lines, Grade 8 = 6 lines). For tap drill sizes across the full metric and imperial range, see our Tap Drill Size Chart (Metric & Imperial). For metric fastener size and grade references, see our Metric Bolt Size Guide. BSF — British Standard Fine BSF (British Standard Fine) is the fine-pitch companion to BSW, defined in BS 84:1956. It uses the identical 55° Whitworth rounded thread form as BSW, but with a finer pitch at each nominal size. BSF was widely used in British precision engineering applications from the early 1900s until metrication in the 1970s. You'll encounter BSF on: Pre-1970s British vehicle engines — many Jaguar, Rolls-Royce, Triumph, Rover and Leyland engines used BSF for cylinder head studs, cam covers and precision internal fittings where BSW's coarser pitch was considered inadequate British-made aircraft and aviation ground support equipment from the pre-metric era (the aviation industry was a major BSF user) Precision instruments, optical equipment and scientific apparatus manufactured in the UK pre-1970 Some legacy Australian-made machinery that followed British engineering practice BSF is technically obsolete for new manufacturing — there are no active orders or new stock being produced in BSF. Maintenance and restoration of legacy equipment are the only reasons to source BSF fasteners today. BSW vs BSF — Side by Side Nominal Size BSW TPI BSF TPI BSW Tap Drill (mm) BSF Tap Drill (mm) 1/4" 20 26 5.1 5.5 5/16" 18 22 6.5 6.8 3/8" 16 20 7.9 8.3 7/16" 14 18 9.4 9.7 1/2" 12 16 10.5 11.1 9/16" 12 16 11.9 12.5 5/8" 11 14 13.5 14.0 3/4" 10 12 16.5 17.0 1" 8 10 22.25 22.75 Identification tip: BSF and BSW share the same thread angle (55°) and the same nominal sizes. The ONLY reliable way to distinguish them is to count TPI with a thread pitch gauge. A 1/2" thread with 12 TPI is BSW; a 1/2" thread with 16 TPI is BSF. Visually, BSF threads appear finer (closer-spaced crests). Do not attempt to determine this by eye alone. For imperial tap and die sets covering BSW and BSF, see our Imperial Hand Taps collection. ACME and Trapezoidal Threads ACME and Trapezoidal threads are power transmission threads, not fastener threads. Instead of clamping two components together, they convert rotational motion into linear motion — in lead screws, lathes, vices, jacks, valve stems and linear actuators. They look completely different from standard fastener or pipe threads and are very unlikely to be confused with them once you know what to look for. ACME Threads (Imperial) ACME threads are governed by ASME B1.5-1997 (reaffirmed 2024). They use a distinctive 29° thread form (14.5° each side from the thread centreline), producing a wide, flat-topped, visible tooth. ACME is the standard power screw thread used in American and Australian-origin lathes, milling machines, toolroom vices, and lifting jacks. Key ACME characteristics: 29° included thread angle — immediately visible as a wider tooth than fastener threads NOT self-locking — a loaded ACME screw will back-drive under load unless a separate brake or lock is fitted. This is a critical safety consideration for vertical lifting applications Specified as: diameter × pitch in TPI (e.g. 3/4-6 ACME = 3/4 inch diameter, 6 threads per inch) Available in General Purpose (G) and Centralising (C) classes — General Purpose is the workshop standard ACME Size Reference Diameter TPI (Coarse) Pitch (mm equiv.) Typical Application 1/4" 16 1.59 Small instrument screws 5/16" 14 1.81 Light jigs and fixtures 3/8" 12 2.12 Small vice screws 1/2" 10 2.54 Medium vice, clamps 5/8" 8 3.18 Lathe cross-slide screws 3/4" 6 4.23 Lathe lead screws, jack screws 1" 5 5.08 Large vice screws, lifting gear 1-1/4" 5 5.08 Heavy lathe bed traverses 1-1/2" 4 6.35 Screw presses, heavy lifting 2" 4 6.35 Large screw jacks, arbor presses Trapezoidal (Tr) Threads — The Metric Equivalent The metric equivalent of ACME is the Trapezoidal thread, designated as Tr and standardised in ISO 2901–2904. The thread form is similar in purpose to ACME but uses a 30° included thread angle (slightly steeper flanks than ACME's 29°) and metric sizing. For example: Tr 20 × 4 = 20mm diameter, 4mm pitch. Feature ACME Trapezoidal (Tr) Standard ASME B1.5-1997 (R2024) ISO 2901–2904 Thread angle 29° 30° Sizing system Imperial (inches, TPI) Metric (mm diameter × mm pitch) Interchange NOT interchangeable — different angle + different sizing system Common on US/AU-origin lathes, vices, jacks European CNC machines, metric lead screws Self-locking? No No Identification: Both ACME and Tr threads are immediately recognisable by their wide, flat-topped tooth profile. If the machine is marked in imperial, it's ACME. If metric, it's Tr. Do not attempt to use ACME taps or dies on a Tr screw — the 1° angle difference and metric pitch will destroy the thread. Thread Selection Guide — Which Standard for Your Application? Choose the right thread standard before you cut, tap or order. Retrofitting is expensive. Application Correct Standard Reason New Australian manufacturing — any fastener Metric M-series (coarse) AS 1275 default; off-the-shelf stock widely available Hydraulic and compressed-air fittings in AU BSP (BSPP or BSPT) Australian/Commonwealth default for fluid systems Plumbing — water and gas BSPT Tapered thread self-seals with PTFE tape US-import equipment fitting or repair NPT American equipment default for pipe threads US-import fasteners or machinery repair UNC (general) or UNF (precision) American fastener standard; match existing thread standard Pre-1970s Australian/British machinery repair BSW (general) or BSF (precision) Match existing thread; check with pitch gauge first Lathe, vice, jack or linear actuator lead screw ACME (imperial) or Tr (metric) Power transmission thread; match the machine's original spec High-vibration or thin-wall precision fastening Metric fine or UNF Finer pitch = better vibration resistance and thread engagement How to Identify a Thread by Sight (and Three Tools That Help) If you've inherited a fitting with no markings and need to know what it is, work through this checklist: Pipe or fastener? If it has a bore (it's hollow), it's almost certainly a pipe thread — BSP or NPT. If it's a solid stud, bolt or screw, it's a fastener thread — UNC, UNF, BSW, or metric M-series. Tapered or parallel? Run a straight edge along the thread. If the OD reduces as you move along the thread, it's tapered (BSPT or NPT). If it's straight, it's parallel (BSPP, UNC, UNF, BSW, M). Check the thread angle with a thread angle gauge. 55° = British heritage (BSP, BSW). 60° = American or metric (NPT, UNC, UNF, M). Measure the pitch with a thread pitch gauge. Compare against the size tables above to confirm the standard. Measure the OD with calipers and cross-reference against the relevant table. The three tools that make this fast: Thread pitch gauge (metric and imperial blade sets) — slide each blade against the thread until one matches Caliper — measure the male OD Thread identification chart — laminated reference card with the common sizes (you've effectively got one above) See our Screw Pitch Gauges collection for thread identification gauges. Are BSP and NPT Interchangeable? (No — Here's Why) No. BSP and NPT are not interchangeable, even though they share several nominal sizes and the same 1:16 taper on the tapered variants. The reasons they don't mesh: Thread angle differs. BSP is 55°, NPT is 60°. The peaks and valleys of the threads have different geometry, so even when they bind for a few turns, only the very tips of the threads contact — there's no real engagement to seal against. TPI is different at most sizes. 1/8" BSP is 28 TPI; 1/8" NPT is 27 TPI. 1/4" BSP is 19 TPI; 1/4" NPT is 18 TPI. 1" BSP is 11 TPI; 1" NPT is 11.5 TPI. The pitch mismatch compounds with the angle mismatch. The thread form differs. BSP has rounded crests and roots (Whitworth form). NPT has flat crests and rounded roots. Even where the angle and TPI happen to match, the form difference means partial-only engagement. The only sizes where BSP and NPT share both TPI AND nominal size are 1/2" and 3/4" (both 14 TPI). At these sizes you'll get further before the angle mismatch reveals itself — which is exactly why these sizes cause the most cross-thread damage in workshops. Always identify the standard before assembly. Don't trust "looks close enough". Sealing Tapered vs Parallel Threads How you seal a thread depends on whether it's tapered or parallel — and getting this wrong is one of the most common causes of leaking fittings in industrial workshops. Tapered threads (BSPT, NPT) The metal-to-metal taper IS the seal. As you tighten, the male wedges into the female and the threads deform slightly to fill voids. PTFE tape (typically 3-5 wraps in the direction of thread engagement) or a thread sealant like Loctite 567 / 577 fills micro-voids and stops capillary leaks, but the seal is fundamentally mechanical. Tradesperson rules: Wind PTFE tape clockwise looking down the male thread (so tightening winds the tape on, not off) Don't apply tape to the first thread — leave it bare to avoid tape entering the system Hand-tight + 1-2 wrench turns is usually enough; over-tightening cracks fittings Liquid sealants like Loctite 577 are often preferred over PTFE for hydraulic applications because they don't shred Parallel threads (BSPP, UNC, UNF, metric) The thread itself does NOT seal. You need a separate sealing element: Bonded seal washer (Dowty) — under the fitting head, the rubber bond compresses against a flat seat O-ring — seated in a port face groove or against a flat sealing face Copper/aluminium crush washer — single-use, deforms to seal Flat fibre or rubber washer — for lower-pressure applications Wrapping PTFE tape around a BSPP male thread and screwing it into a BSPP port without a Dowty or O-ring is a leak waiting to happen. The threads simply do not have the geometry to seal themselves. For sealing products see our Thread Sealants collection. Common Conversion Mistakes That Destroy Fittings The most expensive errors we see at AIMS Industrial — collected from years of customer calls: BSP male into NPT female on imported hydraulic gear. Customer assumes the fitting is BSP because it looks like all the others; equipment is American and the port is NPT. After 3 turns it binds. Customer tightens harder. The casting cracks. Replacement pump housing: $400-2,000. NPT male into BSP female on Aussie plumbing. Reverse of above. Common with imported pneumatic tools forced onto BSP shop air lines. Slight angle mismatch means it leaks under pressure no matter how much PTFE tape you wrap. BSPP forced into BSPT (or vice versa) without realising the receiver is the other one. Same nominal size, same 55° angle, same TPI — but one is parallel and one is tapered. Parallel-into-tapered won't reach full engagement. Tapered-into-parallel won't seal because nothing wedges. UNC bolt into UNF nut. Same nominal diameter, same 60° angle, different TPI. The bolt will start, then bind or strip the nut after a few turns. 1/4" BSW bolt into 1/4" UNC nut. Same TPI (both 20), same nominal size, different angle (55° vs 60°). Forces will work but the joint has only partial thread engagement and minimal preload capacity. Common on classic-vehicle restorations. M10 bolt into 3/8" UNC hole. Nearly the same nominal diameter (10mm vs 9.525mm), different angle (60° matches but pitches don't — 1.5mm vs 1.59mm). Will bind partway in. The fix for all of these: identify before you tighten. A 30-second check with a thread pitch gauge prevents a $400 mistake. Tools You Need to Get Thread Identification Right The basic tradie kit for any workshop dealing with multiple thread standards: Thread pitch gauge (metric and imperial sets) — see Screw Pitch Gauges Caliper for measuring OD and pitch diameter Tap and die set covering the standards you work with — see Taps, Imperial Hand Taps, Metric Spiral Point Taps Pipe dies for the pipe standards you encounter — see Dieheads for Pipe Machines Thread sealants — see Thread Sealants AIMS Thread-Standard Product Cross-Reference Sourcing fittings and tools for each standard from AIMS Industrial: BSP fittings: Brass Fittings, Iron Pipe Fittings, Pipe Fittings, Hose Fittings & Couplings Imperial taps (UNC / UNF / BSW / BSP): Imperial Hand Taps, Imperial Spiral Flute Taps Metric taps: Metric Spiral Point Taps Stainless fasteners (UNC, UNF, metric): Stainless Fasteners Thread identification: Screw Pitch Gauges Thread sealants: Thread Sealants Full threading range: Threading Collection Sutton Tools (Australian-made cutting tools): Sutton Tools Related Reference Articles Tap Drill Size Chart — Metric & Imperial (the size data behind this article) Metric vs Imperial Fastener Reference Guide Drill Bit Size Chart — Metric, Imperial, Fractional Metric Bolt Size Guide Loctite 577 Pipe Sealant Guide Spiral Wound Gasket Guide Imperial/SAE Bolt Torque Chart — tightening torques for UNC and UNF Grade 2, 5 and 8 fasteners Frequently Asked Questions What is BSP thread? BSP (British Standard Pipe) is the dominant pipe thread standard in Australia, the UK and Europe. It uses a 55° rounded Whitworth thread form. There are two variants: BSPP (parallel — needs a separate seal) and BSPT (tapered — seals via metal-to-metal interference). Defined by ISO 228 (BSPP) and ISO 7-1 (BSPT). What is NPT thread? NPT (National Pipe Taper) is the American pipe thread standard, governed by ANSI/ASME B1.20.1. It uses a 60° symmetric thread form on a 1:16 taper. The thread self-seals via the taper. You'll see NPT in Australia mostly on US-imported equipment — compressors, hydraulic pumps, pneumatic tools. What is the difference between BSP and NPT? BSP and NPT differ in three critical ways: thread angle (BSP is 55°, NPT is 60°), thread form (BSP has rounded crests, NPT has flat crests), and pitch (TPI differs at most nominal sizes). They are not interchangeable, even where they share a nominal size. Forcing one into the other will cross-thread, leak, or crack the fitting. Is BSP the same as NPT? No. BSP and NPT share neither thread angle, thread form, nor TPI at most sizes. They look similar because they share the 1:16 taper on the tapered variants (BSPT and NPT). The visual similarity is the cause of most cross-threading damage in Australian workshops. What is the difference between BSPP and BSPT? BSPP (Parallel) and BSPT (Tapered) share the same 55° Whitworth thread form, the same TPI, and the same nominal sizes. The difference is the thread profile along the pipe length: BSPP runs straight, BSPT runs on a 1:16 taper. BSPP needs a separate seal (bonded washer, O-ring); BSPT self-seals via the taper plus PTFE tape or thread sealant. How do I identify a BSP thread? Use a thread pitch gauge to measure pitch and a 55° thread angle gauge. A 1/2" BSP male thread has an OD of approximately 21mm and 14 TPI — matching neither metric M-series nor any UN-series fastener at 1/2" nominal size. If the OD is significantly larger than the nominal size suggests, you're probably looking at a BSP pipe thread. How do I identify an NPT thread? Same process as BSP — pitch gauge, thread angle gauge — but you're looking for a 60° angle (NPT specific) and a tapered profile. 1/2" NPT is 14 TPI with a male OD of about 21.2mm at the gauge plane. Compare against the NPT table in this article. Note: NPT and BSPT at 1/2" share TPI (both 14) — distinguish by thread angle (60° vs 55°) and the slightly larger NPT OD. What is UNC thread? UNC (Unified National Coarse) is the American imperial fastener thread standard for general-purpose work. It uses a 60° symmetric thread form. Example: 1/2"-13 UNC means 1/2 inch nominal diameter, 13 threads per inch. Defined by ANSI/ASME B1.1. What is UNF thread? UNF (Unified National Fine) is the fine-pitch counterpart to UNC. Same 60° thread form, but more threads per inch — for example 1/2"-20 UNF has 20 TPI (compared to 13 TPI for 1/2" UNC). Used where precision, vibration resistance or thin-wall thread engagement matters: aerospace, motorsport, hydraulic fittings. What is the difference between UNC and UNF? UNC has fewer threads per inch (coarser pitch); UNF has more threads per inch (finer pitch). UNC is faster to assemble and more tolerant of dirty conditions. UNF gives finer adjustability, better vibration resistance and more thread engagement per length of thread. They are not interchangeable at the same nominal size. Is BSW the same as BSP? No, but they share the 55° Whitworth thread form. BSW (British Standard Whitworth) is a fastener thread standard. BSP (British Standard Pipe) is a pipe thread standard. The sizing conventions and applications are different. BSW is for bolts and studs; BSP is for fittings on pipes and bores. Can I screw a BSP fitting into an NPT thread? You can start it, but you should not commit to it. BSP and NPT have different thread angles (55° vs 60°), different TPIs at most sizes, and different thread forms. The fit will be partial, the seal will leak under pressure, and over-tightening to force a seal will crack the casting. Use the correct standard for the receiving thread — always. What sealant should I use on BSPT threads? BSPT is a tapered thread that self-seals. PTFE tape (3-5 wraps clockwise looking at the male thread, leaving the first thread bare) is the common workshop choice. For hydraulic applications, liquid sealants like Loctite 567 or Loctite 577 are often preferred because they don't shred under high pressure. Don't over-wrap — excess tape can split fittings. What sealant should I use on BSPP threads? BSPP is parallel — the thread itself does not seal. You need a separate sealing element such as a bonded seal washer (Dowty), an O-ring seated in a port face, or a copper crush washer. Wrapping PTFE tape on a BSPP male and trying to seal it is a common mistake; it will leak under pressure because the thread provides no wedging action. How can I tell if a thread is metric or imperial? Measure the pitch with a thread pitch gauge. Imperial threads are specified in TPI (threads per inch); metric threads are specified in mm pitch. If your gauge blades match a metric pitch (0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5, 3.0mm), it's metric. If they match a TPI value (16, 18, 20, 24, 28 etc.), it's imperial. The 60° vs 55° angle test also helps — metric is always 60°, BSP and BSW are 55°. Why do BSP and NPT both use a 1:16 taper? The 1:16 taper (3/4 inch reduction per foot of length) was chosen independently by both standards because it gives a good balance of thread engagement and sealing force without requiring excessive turns to tighten. The shared taper is the reason BSPT and NPT look similar at a glance — but the underlying thread angles (55° vs 60°) make them mechanically incompatible. Cross-reference our Tap Types guide when picking between taper, plug, bottoming, gun and spiral flute taps.People Also Ask — Thread Identification Q: How do I identify an unknown thread without a gauge? Without a thread pitch gauge, start with the basics: check if the thread is on a pipe/bore (likely BSP or NPT) or a solid fastener (metric, UNC/UNF, BSW/BSF). Then look at the thread angle visually — 55° threads have noticeably rounded crests and a gentler profile compared to 60° threads. Count the number of thread crests visible over a 25mm (1 inch) length using a ruler to estimate TPI. Compare against the size tables in this guide. A proper thread pitch gauge is a $30–50 investment that pays for itself on the first identification job — see our Screw Pitch Gauges collection. Q: What is the main difference between metric and imperial threads? The main differences are sizing convention and pitch specification. Metric threads are specified by diameter in millimetres and pitch in millimetres (M10 × 1.5 = 10mm diameter, 1.5mm between crests). Imperial threads are specified by diameter in fractions of an inch and pitch in TPI — threads per inch (1/2"-13 UNC = 1/2 inch diameter, 13 threads per inch). Both metric and UNC/UNF use a 60° thread form, so angle alone won't distinguish them. You need the pitch gauge and calipers. A 10mm diameter is metric; 3/8" (9.525mm) is the nearest imperial equivalent — and the pitches don't match. Q: Can I use a metric bolt in an imperial nut? No — not reliably. While some metric and imperial sizes are close in diameter, the pitch (thread spacing) doesn't match. For example, M10 × 1.5 and 3/8"-16 UNC have similar diameters (10mm vs 9.53mm) but different pitches (1.5mm metric vs 1.59mm imperial). They'll start but bind partway in, and any force will strip the nut. The one-turn rule applies: if a bolt doesn't thread in cleanly by hand for at least 3–4 turns, it's the wrong thread. Never use a wrench to force a thread that won't start cleanly by hand. Q: What thread standard do Australian plumbers and hydraulic fitters use? Australian plumbers and hydraulic fitters use BSP (British Standard Pipe) as the default. BSPT (tapered) is used for water, gas and most pressure fittings where the thread itself creates the seal. BSPP (parallel) is used for hydraulic systems where an O-ring or bonded seal provides the sealing function. You'll only encounter NPT on US-imported equipment — it is not the Australian default and is not used in domestic plumbing. If you're buying fittings from an Australian supplier for an Australian installation, specify BSP unless the existing fittings are confirmed NPT. Q: What is the difference between BSF and BSW threads? BSF (British Standard Fine) and BSW (British Standard Whitworth) share the same 55° Whitworth thread form and nominal size range, but BSF has more threads per inch at each size. For example, 1/2" BSW is 12 TPI and 1/2" BSF is 16 TPI. They are not interchangeable at the same nominal size. BSW was used for general structural fastening; BSF was used where vibration resistance or precision was required — engine internals, aviation hardware, precision instruments. Both are obsolete for new manufacturing. Identify which you have using a thread pitch gauge before ordering replacement fasteners. People Also Ask — Thread Identification — BSP, NPT, Metric & More Q: What is the difference between BSP and NPT threads? BSP (British Standard Pipe) uses a 55° thread angle with a parallel thread form in the most common variant (BSPP), while NPT (National Pipe Taper) uses a 60° thread angle with a tapered thread that self-seals. The two standards are not interchangeable — forcing a BSP fitting into an NPT port risks cross-threading and leaks even when the threads initially appear to engage. Q: How do I identify a thread type without gauges? Measure the thread pitch (distance between crests) with a thread pitch gauge or by counting threads over a known length, then measure the outside diameter. Compare these values against size charts for BSP, NPT, and metric. The thread angle — 55° for BSP and Whitworth, 60° for NPT, UNC, and metric — also helps distinguish families, though a profile gauge makes this faster and more reliable. Q: Can BSP and NPT fittings be mixed? No. BSP and NPT fittings appear similar in some sizes and have nearly identical thread counts in certain dimensions, but the different thread angles (55° vs 60°) and pitch values mean they will not seal correctly together. Using a BSPP fitting in an NPT port, or vice versa, typically produces a leak-prone connection even when it initially feels tight. Q: What does BSPP mean compared to BSPT? BSPP is British Standard Pipe Parallel — the thread maintains the same diameter along its length and relies on a face seal or O-ring for sealing. BSPT is British Standard Pipe Taper — the thread tapers and the taper itself creates a seal in conjunction with PTFE tape or thread sealant. Most hydraulic and pneumatic systems use BSPP with an O-ring face seal. Q: How do I measure thread pitch? For metric threads, pitch is measured directly as the distance between adjacent crests in millimetres. For imperial threads (BSP, NPT, UNC), pitch is expressed as threads per inch (TPI) — count the crests over one inch of thread length. Thread pitch gauges are the fastest and most reliable method, with common sets covering both metric and imperial standards. Browse metric thread forming taps at AIMS Industrial for application support and stock confirmation. AIMS Industrial stocks taper pipe reamers — see the full range for trade and industrial use.What thread angle do BSP, NPT and UNC threads use? BSP threads use the Whitworth 55-degree thread form with rounded roots and crests; NPT and the Unified series (UNC and UNF) use a 60-degree angle. That difference in angle is one reason a BSP and an NPT fitting of the same nominal size don't seal properly together even when the diameters look close. Always match the thread form, not just the nominal size. See pipe fittings. What is an ACME thread used for? ACME is a trapezoidal thread form (29-degree flanks) designed to transmit motion and load rather than to fasten or seal — used on lead screws, vices, clamps, valve stems and machine actuators. Its broad, square-ish profile carries axial load efficiently and resists wear better than a V-thread. It is not interchangeable with any fastening or pipe thread. Is Whitworth (BSW) thread still used? British Standard Whitworth (BSW) and British Standard Fine (BSF) are the older 55-degree imperial fastener threads, largely superseded by metric and Unified threads but still found on older British machinery, vintage vehicles and some legacy plant in Australia. Identify them with a thread gauge — the 55-degree form and Whitworth pitches set them apart from UNC and UNF. For hydraulic and pipe thread selection see our hydraulic fittings guide.Once the thread standard is confirmed, whether it needs a sealant at all — and which grade — depends on whether it's a tapered pressure thread or a parallel one sealing on a washer. See our Loctite Thread Sealant Guide for grade selection by thread type.
Read moreTap & Die Guide: Cutting Threads
How to Cut Threads with a Tap & Die — Quick Reference The seven-step process for cutting accurate threads using hand taps and dies. Select the correct tap drill size — match the drill diameter to the tap from a tap drill chart (e.g. M6 × 1.0 = 5.0 mm drill). Drill the pilot hole square and clean — use cutting fluid; deburr both sides of the hole. Use cutting fluid — never tap dry. Cutting fluid prevents tap breakage and gives clean threads. Start with a taper tap — 7–10 cutting threads on the leading edge to ease into the hole. Turn forward two turns, then back a quarter turn — clears chips and prevents binding. Repeat for full depth. For blind holes, finish with a bottoming tap — fewer leading threads, cuts to the bottom of the hole. For external threads, use a die with a die stock — keep square to the work, apply cutting fluid, same forward-back rhythm. Set this aside as your basic tapping procedure. The detailed sections below cover drill size selection, tap types, common problems and recovery from broken taps. Tap & Die Set Guide: How to Tap Threads & Cut Externals A tap and die set is the standard tool for cutting internal and external screw threads by hand. A tap cuts the female thread inside a drilled hole; a die cuts the male thread onto a rod or bolt shank. Together they cover thread creation, thread repair, and thread restoration across the full range of metric, imperial, and pipe thread standards used in Australian industry, automotive, engineering, and maintenance work. This guide covers how both tools work, how to select the correct drill size before you tap (this is where most threads fail), which tap type to use for through holes versus blind holes, how to cut external threads cleanly, how to choose the right lubricant for the material you are threading, the difference between thread cutting and thread chasing, and the root causes of broken taps and how to prevent them. Contents What are taps and dies? Types of taps: taper, plug, and bottoming Thread standards in Australia Tap drill size: the critical first step How to tap a thread (step by step) How to cut external threads with a die Lubrication by material Thread chasing vs thread cutting Common mistakes and broken taps Frequently asked questions What are taps and dies? A tap is a fluted, hardened steel tool used to cut internal threads inside a pre-drilled hole. The flutes run along the length of the tap body; they provide the cutting edges and allow chips to escape during cutting. The tap is rotated into the hole using a tap wrench or T-handle, and it removes material in a helical pattern to form the thread profile. A die is a hardened circular tool with a central threaded aperture and cutting edges around its inside diameter. It is held in a die stock (a handle with a central hole to seat the die). The die is placed over the end of a rod or bolt shank and rotated to cut an external thread. Most dies are split and adjustable — a small screw allows the aperture to be opened slightly for a first rough pass and then closed to final size for a finishing pass. The tap cuts the nut; the die cuts the bolt. That is the simplest way to remember which does what. Taps and dies are made from one of three materials, depending on application and price point: High-speed steel (HSS): The standard for industrial and professional use. Suitable for steel, aluminium, brass, cast iron, and most engineering materials. Resharpening is possible. HSS is the correct choice for serious workshop use. Carbon steel: Found in cheaper consumer-grade sets. Adequate for occasional soft material use (aluminium, brass, plastic). Unsuitable for stainless steel or repeated hard steel use. Edge life is substantially shorter than HSS. HSS-Co (cobalt HSS): Premium grade for stainless steel, titanium, and high-alloy steels. Higher cost, significantly better performance in hard or abrasive materials. Types of taps: taper, plug, and bottoming Hand taps are produced in three configurations that differ in the amount of lead chamfer — the tapered section at the tip that begins the cutting action. Selecting the correct type for the job prevents the most common beginner failures. Taper tap (also called starting tap) A taper tap has 7–10 threads chamfered at the tip, creating a long, gradual entry. The extended lead distributes cutting load over many teeth, making the tap easy to start square and reducing torque at entry. Taper taps are the correct first choice for starting new threads in any unthreaded hole. They work well in through holes and are forgiving of minor misalignment at the start. Limitation: The long chamfer means the taper tap cannot thread to within 7–10 thread pitches of the bottom of a blind hole. For blind holes requiring full-depth threads, a plug or bottoming tap must follow. Plug tap (also called second tap or intermediate tap) A plug tap has 3–5 chamfered threads at the tip. It can start in an unthreaded hole (useful when a taper tap is not available), cuts threads closer to the bottom of a blind hole than a taper tap, and is the most common general-purpose tap included in standard sets. If a tap and die set includes only one tap per size, it is almost always a plug tap. For most through-hole tapping applications, a plug tap alone is sufficient. For blind holes, use a taper tap first to establish the thread, then follow with a plug tap to deepen it. Bottoming tap (also called third tap or bottom tap) A bottoming tap has only 1–2 chamfered threads. It cannot start in an unthreaded hole — attempting to do so is a reliable way to break the tap. Its sole purpose is to extend threads to within 1–2 pitches of the bottom of a blind hole after a taper and/or plug tap has already cut the thread. If your application requires full-depth threading in a blind hole, the correct sequence is: taper tap → plug tap → bottoming tap. Skipping to the bottoming tap immediately is the single most common cause of tap breakage among beginners. ✅ Which tap to use: quick reference Through hole: Plug tap alone is sufficient. Taper tap first if you want easiest starting. Blind hole, partial depth: Taper tap → plug tap. Blind hole, full depth to bottom: Taper tap → plug tap → bottoming tap. Spiral point (gun) taps Spiral point taps have a modified cutting face that pushes chips forward and down through the hole rather than evacuating them backward. They are faster than hand taps in through holes and are the standard choice for machine tapping. They are not suitable for blind holes — chips pushed to the bottom have nowhere to go and will cause jamming. Spiral flute taps Spiral flute taps have helical flutes that pull chips up and out of the hole, away from the cutting zone. They are the correct choice for blind holes in machine tapping, and are particularly effective in soft, stringy materials like aluminium and stainless steel. Not common in hand tap sets but worth knowing about for production applications. Thread standards in Australia Australia uses three thread standard families in everyday industrial, mechanical, and plumbing applications. Buying the right set and selecting the right tap for the job requires understanding which standard applies to your application. Metric (M) threads The dominant thread standard for fasteners in Australia. All modern machinery, automotive, structural, and most engineering fasteners use metric threads. Metric threads are defined by nominal diameter and pitch: M8×1.25 means 8 mm major diameter, 1.25 mm between thread crests. Coarse pitch is the standard for most fastener applications; fine pitch (e.g., M8×1.0) is used where vibration resistance, thin-wall material, or precise adjustment is required. A metric coarse tap and die set covering M3 to M12 handles the overwhelming majority of general workshop work. Sets extending to M20 cover structural, heavy engineering, and automotive applications. BSP (British Standard Pipe) threads BSP threads are standard for pipe fittings, hydraulic connections, pneumatic fittings, and plumbing in Australia and New Zealand. BSP uses a 55° thread angle (compared to 60° for metric) and thread pitch defined in threads per inch. Two variants exist: BSPP (BSP parallel, also called G thread): Both male and female threads are parallel. The seal is made by a bonded seal washer (Dowty seal) or O-ring at the face, not by the threads. Most common in hydraulic and pneumatic fittings. BSPT (BSP taper): The male thread is tapered (1:16 taper). Sealing is achieved by the taper interference, often supplemented by PTFE tape. Common in plumbing and gas applications. BSP sizes are nominal pipe sizes, not actual thread diameters: a ½" BSP fitting has an actual thread OD of approximately 20.96 mm — considerably larger than ½". This causes persistent confusion when measuring. A dedicated BSP tap and die set is needed for pipe thread work; metric taps will not cut BSP threads even if the diameter appears similar. UNC / UNF imperial threads Unified National Coarse (UNC) and Unified National Fine (UNF) threads are the standard for imperial fasteners, predominantly found in older Australian equipment, American-made machinery, and imported automotive components. UNC/UNF uses a 60° thread angle (same as metric) but pitch is defined in threads per inch rather than millimetres. A ⅜"-16 UNC fastener has a ⅜" major diameter and 16 threads per inch. If your application involves older equipment, American vehicles, or any fastener sold in fractional inch sizing, you need an imperial tap and die set. Metric and imperial taps will not interchange — do not attempt to run an M10 tap into a thread started by a ⅜"-16 die. Tap drill size: the critical first step The most common cause of failed threads — weak engagement, tap breakage, torn threads — is an incorrectly sized pilot hole. Too small, and the tap must remove too much material: cutting torque rises sharply, and the tap breaks or the hole strips. Too large, and thread engagement is shallow: the resulting thread is weak and will strip under load. The standard tap drill size gives approximately 75% thread engagement — the industry benchmark that balances thread strength against cutting torque. At 75% engagement, the thread achieves approximately 98% of the strength of full (100%) thread engagement, while cutting torque is manageable. Going to 65% engagement (0.1–0.2 mm larger drill) is common practice for hard materials (stainless steel, titanium, high-tensile alloys) where reducing tap breakage risk outweighs the marginal strength reduction. Tap drill formula (metric): Tap drill diameter = Nominal diameter − Pitch Example: M10×1.5 → tap drill = 10 − 1.5 = 8.5 mm The following table covers the metric coarse thread sizes most commonly tapped in workshop practice, plus key BSP sizes: Thread size Pitch (mm) Standard tap drill (75% engagement) Reduced engagement drill (65%, hard materials) M3 0.5 2.5 mm 2.6 mm M4 0.7 3.3 mm 3.4 mm M5 0.8 4.2 mm 4.3 mm M6 1.0 5.0 mm 5.1 mm M8 1.25 6.8 mm 6.9 mm M10 1.5 8.5 mm 8.7 mm M12 1.75 10.2 mm 10.4 mm M14 2.0 12.0 mm 12.2 mm M16 2.0 14.0 mm 14.2 mm M20 2.5 17.5 mm 17.7 mm ¼" BSP (BSPP/BSPT) 19 TPI 11.8 mm — ⅜" BSP 19 TPI 15.3 mm — ½" BSP 14 TPI 19.1 mm — ¾" BSP 14 TPI 24.5 mm — 1" BSP 11 TPI 30.5 mm — Always verify tap drill size against the specific tap manufacturer's data before drilling. Variations of ±0.1 mm exist between standards. For critical applications, consult the tap manufacturer's drill size recommendation. How to tap a thread (step by step) Step 1: Mark and centre-punch the hole location Accuracy at this step determines alignment through the entire process. Use a centre punch to dimple the surface at the exact hole location before drilling. The dimple prevents the drill from walking off position and ensures the hole starts where intended. Step 2: Drill the pilot hole to the correct size Use the tap drill size from the table above. Drill the hole square to the surface — a drill press is strongly preferred over a handheld drill for critical applications. Misalignment of even 1–2° will be magnified through the tapping process and produce a crooked thread. For blind holes: drill to a depth equal to the required thread depth plus 3–5 thread pitches of clearance. The tap needs space beyond the thread zone to avoid bottoming out. Mark the required depth on the drill bit with tape. Step 3: Deburr the hole entry After drilling, use a larger drill bit or countersink (held by hand and rotated) to chamfer the top edge of the hole lightly. This removes the sharp burr raised by drilling, provides a lead-in for the tap, and prevents the first thread from being raised above the surface — a common cause of nut/bolt interference. Step 4: Apply cutting lubricant Apply lubricant to the tap before entering the hole. Do not dry-tap any material except cast iron and some plastics. See the lubrication section below for material-specific recommendations. Step 5: Start the tap square This is the most critical step. Place the taper tap at the hole entrance and apply gentle downward pressure while rotating slowly clockwise. After the first 1–2 full turns, the tap is threading itself and no further downward pressure is needed — the thread pitch pulls the tap in at the correct rate. Use a small engineer's square held against the tap body and the work surface to verify the tap is entering square. If it is tilted, back the tap out completely and restart. Tapping a crooked thread cannot be corrected once started. Step 6: Use the forward-back chip-breaking rhythm Advance the tap ¾ to 1 full turn forward, then reverse ¼ to ½ turn. The reverse stroke breaks the chip, preventing the chip mass from packing in the flutes and jamming the tap. This rhythm is non-negotiable in any material that produces continuous chips — steel, stainless, aluminium. In brittle materials (cast iron, brass), chips break naturally and the rhythm is less critical but still good practice. Never force a tap. If resistance increases sharply, back the tap out, clear the chips, re-lubricate, and re-enter. Forcing a tight tap is the second most common cause of breakage after misalignment. Step 7: For blind holes, manage depth carefully Back the tap out completely periodically to clear chips from the flutes. In blind holes, chips cannot fall through — they accumulate in the flutes and at the hole bottom. A tap jammed against a chip mass at the bottom of a blind hole will break. Clear chips every 4–5 full rotations in blind holes, more frequently in soft materials that produce long, stringy chips. Step 8: Follow with plug and bottoming taps if required Once the taper tap has completed its depth, follow with a plug tap using the same technique to deepen the thread, and then a bottoming tap if full-depth threading to the hole bottom is required. Re-lubricate between each tap. Step 9: Clean the threaded hole Before installing any fastener, clear the tapped hole of chips and cutting fluid. Compressed air into the hole (wear eye protection), followed by a thread cleaning brush or a bolt with the shank wrapped in a rag, removes residual chips. A chip in the thread will prevent a fastener from seating fully and can strip the thread on installation. How to cut external threads with a die Cutting external threads with a die follows similar principles to tapping — correct preparation, starting square, and the forward-back rhythm — with a few specific differences. Prepare the rod or bar end The rod must be the correct diameter for the thread being cut. For metric threads, the rod diameter should equal the nominal thread diameter within a tolerance of −0.05 to −0.15 mm. A slightly undersize rod produces a correct fit; a rod exactly at nominal diameter may be too tight for the die to start. File or turn a 15–20° chamfer on the end of the rod — this gives the die a lead-in and prevents the die from splitting the first thread. Set the die in the stock Place the die in the die stock with the chamfered (lead) side facing down toward the rod end. Most dies are marked on one face — this marked face faces up in the stock. The three adjustment screws in the stock seat the die centrally. For adjustable split dies, open the die slightly (loosen the centre screw, tighten the two outer screws) for the first rough pass. Start the die square As with tapping, starting square is critical. Place the die flat against the rod end and apply downward pressure while rotating clockwise. If the rod is held in a vice, orient the die stock handles vertically and use them as a visual reference. After 2–3 threads are engaged, the die is self-pulling and no downward pressure is required. Use the forward-back rhythm and lubricate The same ¾ turn forward, ¼ turn back chip-breaking rhythm applies. Lubricate the die and rod throughout. Dies are more susceptible to chip packing than taps because the die surrounds the material — chips have less room to escape. Finish to size After the first rough pass, back off the die and close it to final size by reversing the adjustment (tighten the centre screw, loosen the outer screws). Run the die through again to cut the threads to full depth and proper fit. Check fit with a nut: the nut should thread freely by hand with no perceptible wobble or binding. Lubrication by material Cutting lubrication reduces friction, removes heat, aids chip evacuation, improves thread finish, and extends tool life. "Any oil" is not adequate — the correct lubricant for the material being threaded makes a measurable difference in both tool life and thread quality. Material Recommended lubricant Notes Mild steel Neat cutting oil or sulphurised threading oil Sulphurised oils (e.g., pipe threading oil) are particularly effective for steel — the sulphur reacts with the steel surface to reduce friction. Do not use on copper or brass (stains). Stainless steel Heavy-duty tapping paste or sulphurised oil Stainless work-hardens rapidly when dry. Inadequate lubrication causes the tap to rub rather than cut, generating heat that hardens the surface and seizes the tap. Do not rush, do not dry-tap. Aluminium Kerosene, WD-40, or purpose-made aluminium tapping fluid (Tap Magic) Aluminium is soft and sticky — it loads up in the flutes rapidly without lubrication. Kerosene is the traditional workshop choice. Dedicated aluminium tapping fluids provide better chip evacuation and finish. Cast iron Dry — no lubricant Cast iron is self-lubricating due to its graphite content. Cutting oil can cause chips to clump and jam the tap. Blow chips clear with compressed air between passes. Brass / bronze Light cutting oil or dry Brass cuts freely with or without lubricant. Light oil improves finish. Avoid sulphurised oils — sulphur stains and can react with copper alloys. Titanium / high-alloy steel Heavy sulphurised oil or specialist tapping paste These materials are hard, work-harden aggressively, and generate significant heat. Use HSS-Co taps, reduce engagement to 65%, and apply generous lubrication. Take the chip-break rhythm seriously — taps break easily in titanium. Plastic / nylon Dry or light oil Most plastics tap dry. Some engineering plastics (HDPE, nylon) benefit from a very light oil. Avoid heavy cutting fluids — they can swell or degrade some polymers. Thread chasing vs thread cutting Thread chasing and thread cutting are fundamentally different operations performed by different tools. Confusing them — specifically, using a standard tap or die to "clean up" a damaged thread — is one of the most damaging mistakes in thread repair work. Thread cutting: creating new threads A standard tap or die cuts new threads by removing material to form the thread profile. When used in an unthreaded hole or on an unthreaded rod, this is correct use. When used to "clean" or "restore" a thread that already exists, a standard tap or die removes a small amount of additional material on every pass — leaving the thread slightly oversized on a bolt or undersized in a nut. The result is a loose, weakened thread that will strip more easily than the original. Thread chasing: restoring existing threads A thread chaser is a tool specifically designed to restore damaged or corroded threads without removing material. Chasers have a relieved profile and work by re-forming and cleaning existing thread crests rather than cutting new material. A thread chaser run through a rusty or slightly burred thread restores it to its original profile — the fit with a mating fastener is preserved. For bolt threads, rethreading dies or thread file sets (files with thread profiles on each face) perform the same function on external threads. For nut or tapped hole threads, spark plug thread chasers are common in automotive use; more general thread tap chasers (also sold as "re-tap" tools) are available in metric and BSP. When to use which: Hole with no threads, or thread so badly stripped it needs to be recut → standard tap (consider a thread insert/Helicoil if the material is thin or soft) Existing thread that is corroded, galled, burred, or has a damaged crest → thread chaser Bolt thread that is lightly damaged or has paint/rust buildup → rethreading die or thread file Common mistakes and broken taps Broken taps are the most costly mistake in tapping work — extracting a broken tap from a blind hole in a critical component can be more expensive than replacing the component entirely. All tap breakage has a root cause that could have been prevented. If a tap has already broken, see our Broken Tap Removal Guide for the six recovery methods. If the parent thread itself is damaged from a broken tap, stripped fastener, or repeated cycling, see our Stripped Thread Repair Guide covering Recoil and Helicoil wire inserts, TimeSert solid bushings, and Keysert locking inserts. 1. Wrong pilot hole size Drilling too small is the direct cause of excessive cutting torque. At 75% thread engagement the tap has enough material to cut cleanly; below this, cutting force rises non-linearly and the tap is increasingly likely to seize or snap. Always use a tap drill chart — never estimate the hole size. 2. Misalignment at entry This is the number one cause of tap breakage in precision work. A tap entering even 2–3° off square will be progressively stressed as it advances. The threads on one side are cut deeper than the other; the tap body is placed in bending stress in addition to torsional stress. Use a drill press for pilot holes. Use an engineer's square to verify the tap at entry. A tap guide — a simple jig that holds the tap perpendicular to the surface — is inexpensive and eliminates this failure mode entirely. 3. No chip-breaking rhythm Tapping straight through without reversing — especially in blind holes or with deep cuts in steel — allows chips to pack into the flutes. Packed flutes jam, torque spikes, and the tap breaks. The forward-back rhythm is not optional; it is the technique. 4. Bottoming out in a blind hole A bottoming tap driven to the base of a blind hole with chips still present will shear off cleanly. Know your hole depth, mark the tap with tape at the appropriate depth, and back out to clear chips before reaching the bottom. 5. Inadequate or wrong lubricant Dry tapping in steel or stainless is a reliable way to break a tap quickly. In stainless, the surface work-hardens under the tap's rubbing face within seconds of dry contact. Always lubricate, and use the correct lubricant for the material. 6. Using a worn or damaged tap HSS taps have a finite service life. A tap with chipped cutting edges or worn flute geometry cuts poorly, generates heat, and is structurally weakened. Inspect taps before use under good light. If a cutting edge is chipped or a flute is cracked, discard the tap. The cost of a new tap is always less than the cost of extracting a broken one. ⚠️ If you break a tap in a workpiece Options in order of destructiveness: (1) tap extractor tool — only works on taps not fully broken below the surface; (2) EDM (electrical discharge machining) — the standard professional method for broken taps in critical components; it burns the tap out without affecting the parent material; (3) drilling out — only possible if the tap is smaller than the next drill size that can be accommodated, and even then risks damaging the hole. For broken taps in critical or expensive components, take the part to a machine shop with EDM capability before attempting destructive extraction. Frequently asked questions What is a tap and die set used for? A tap and die set is used to cut screw threads. The tap cuts internal (female) threads inside a drilled hole — allowing a bolt or machine screw to thread into it. The die cuts external (male) threads onto a rod or bolt shank. Together they are used to create new threads, repair stripped or damaged threads, and restore corroded or galled fasteners. Common applications include workshop fabrication, automotive repair, machinery maintenance, and plumbing and pipe fitting work. What is the difference between a taper, plug, and bottoming tap? The three tap types differ in the lead chamfer at the tip. A taper tap has 7–10 chamfered threads — the long lead makes it easy to start square and distributes cutting load, but it cannot thread to within 7–10 pitches of the bottom of a blind hole. A plug tap has 3–5 chamfered threads — it is the general-purpose tap for most jobs. A bottoming tap has only 1–2 chamfered threads — it cannot start in an unthreaded hole but can extend threads to the very bottom of a blind hole after the taper and plug taps have done their work. For blind holes requiring full-depth threads, use all three in sequence: taper, then plug, then bottoming. What size drill do I use before tapping? The tap drill size equals the nominal thread diameter minus the thread pitch for metric coarse threads. Common sizes: M6×1.0 requires a 5.0 mm drill; M8×1.25 requires 6.8 mm; M10×1.5 requires 8.5 mm; M12×1.75 requires 10.2 mm. This gives approximately 75% thread engagement, which is the standard recommended for most materials. For hard materials like stainless steel or titanium, drill 0.1–0.2 mm larger to reduce cutting torque and tap breakage risk — the thread strength reduction is marginal. Always verify with the tap drill chart included with your set or the tap manufacturer's data. What is a BSP tap and die set? A BSP (British Standard Pipe) tap and die set cuts the pipe thread standard used for plumbing, hydraulic, pneumatic, and gas fittings in Australia and New Zealand. BSP threads have a 55° thread angle (unlike the 60° of metric and UNC threads) and pitch measured in threads per inch. BSP taps and dies will not interchange with metric tools even when sizes appear similar. Two BSP types exist: BSPP (parallel, used with a bonded seal or O-ring) and BSPT (tapered, seals by thread interference). A combined BSP set covering ⅛" to 1" handles most workshop and plumbing applications. How do I use a die to cut external threads? Chamfer the rod end at 15–20° to give the die a lead-in. Mount the die in the die stock with the chamfered face of the die toward the rod. Apply cutting fluid. Place the die flat on the rod end and rotate clockwise with gentle downward pressure until 2–3 threads engage — after this the die is self-pulling. Use the forward-back chip-breaking rhythm throughout. For adjustable split dies, run the die open for the first pass, then close to final size and run through again for a correct fit. Check with a mating nut — it should thread freely by hand with no wobble. What lubricant should I use when tapping? The correct lubricant depends on the material: use neat cutting oil or sulphurised threading oil for mild steel; heavy tapping paste or sulphurised oil for stainless steel (which work-hardens rapidly without lubrication); kerosene or a dedicated aluminium tapping fluid for aluminium; no lubricant for cast iron (it is self-lubricating and oil causes chip clumping); light oil or dry for brass and bronze. Do not use water-soluble coolants as a substitute for tapping oil — they are designed for flood cooling, not boundary lubrication under slow sliding contact. What is the difference between thread cutting and thread chasing? Thread cutting creates new threads by removing material. Thread chasing restores existing threads without removing material. Using a standard tap or die to clean up a damaged thread removes additional metal and leaves the thread slightly oversize or undersize, producing a looser, weaker fit than the original. Thread chasers — specifically designed tools with a relieved profile — re-form and clean thread crests without cutting new material, preserving the original thread dimensions. For damaged or corroded threads on existing fasteners and fittings, use a thread chaser, not a standard tap or die. Why do taps break and how do I prevent it? Taps break for six main reasons: pilot hole too small (excessive cutting torque); misalignment at entry (bending stress on the tap body); no chip-breaking rhythm (chips pack and jam); bottoming out in a blind hole; inadequate or wrong lubricant; and using a worn or chipped tap. Prevention: always use the correct tap drill size, verify alignment at entry with an engineer's square, use the forward-back rhythm consistently, mark depth on the tap when working in blind holes, lubricate correctly for the material, and inspect taps before use. These six habits eliminate the vast majority of tap breakage. Can I use metric taps on imperial threads or vice versa? No. Metric and imperial (UNC/UNF) threads have different pitches, different diameters, and the same 60° thread angle — which makes them appear interchangeable but they are not. An M10×1.5 tap and a ⅜"-16 UNC tap are close in diameter (10 mm vs 9.53 mm) but have different pitches and diameter. Starting a metric tap in an imperial thread, or vice versa, will cross-thread and destroy both the tap and the workpiece. Always identify the thread standard before selecting a tap. BSP threads are a further separate standard with a 55° angle — completely non-interchangeable with either metric or UNC. How do I identify an unknown thread? Identifying an unknown thread requires two measurements: the thread pitch and the outside diameter. Use a thread pitch gauge (a set of combs with different pitch profiles) to identify the pitch by finding the comb that fits perfectly with no rocking. Then measure the outside diameter with a vernier calliper or micrometer. With pitch and diameter, cross-reference a thread identification chart to determine the standard and size. For pipe threads, note that BSP nominal sizes do not correspond to actual diameters — a ½" BSP thread has an OD of approximately 21 mm, not 12.7 mm. What is a thread insert and when should I use one? A thread insert (commonly sold as Helicoil or Time-Sert) is a helical coil or solid insert of hardened steel that is fitted into a tapped hole to provide a stronger, more durable thread than the parent material alone. Thread inserts are used when: the parent material is too soft to hold a thread reliably (aluminium, magnesium, plastic); a thread has been stripped and the hole cannot be replaced; a metric thread needs to be added to a location previously held a different thread; or when thread strength must be increased beyond what the parent material can provide. Installing a thread insert requires drilling the hole oversize to a specific insert tap drill, tapping with a special insert tap, and pressing or winding the insert in with an installation tool. Metric or imperial — which tap and die set should I buy? For general Australian workshop use, buy a metric set first. Modern machinery, automotive, structural fasteners, and new fabrication in Australia are overwhelmingly metric. A metric coarse set covering M3 to M12 (or M3 to M20 for heavier work) will handle the majority of applications. If you work on older equipment, American vehicles, or agricultural machinery with imperial fasteners, add a UNC/UNF imperial set. If you do any plumbing, hydraulic, pneumatic, or gas fitting work, a BSP set is essential and cannot be substituted with metric tools. High-quality HSS sets from Sutton Tools (Australian-made), Gearwrench, Irwin, or LPR Toolmakers are appropriate for professional workshop use. Avoid carbon-steel sets for anything beyond occasional soft-material use. AIMS Industrial stocks tap and die sets in metric, imperial, and BSP across professional HSS and HSS-Co grades. For thread repair kits, individual tap sizes, and cutting fluids, contact our team. People Also Ask — Taps and Dies for Thread Cutting Q: What is the difference between a taper tap, plug tap and bottoming tap? All three cut the same thread profile but differ in their starting taper. A taper tap has a long lead taper (7–10 threads), making it the easiest to start in a hole and align correctly — used first to start a thread. A plug tap has a shorter taper (3–5 threads) and is used for general-purpose tapping once started. A bottoming tap has almost no taper and cuts threads to the very bottom of a blind hole. The correct sequence for a blind hole is taper → plug → bottoming tap. Q: What size drill bit should I use before tapping a thread? The tap drill size equals the thread's nominal diameter minus one pitch. For an M8 × 1.25 tap, the drill is 8 − 1.25 = 6.75mm (typically rounded to 6.8mm). Drill too small and the tap breaks; drill too large and the thread has insufficient engagement depth. Tap drill charts — available on the AIMS threading guide — list correct drill sizes for all metric and UNF/UNC thread sizes. Always use the correct drill and cutting fluid for the material being tapped. Q: What is a spiral point versus spiral flute tap? A spiral point tap (also called a gun tap) has a straight flute with an angled cutting face that pushes chips ahead of the tap down into through-holes. It is fast and effective in through-hole tapping, especially in ductile metals. A spiral flute tap has helical flutes that draw chips back up out of the hole — essential for blind holes where chips cannot be pushed through. Using a spiral point tap in a blind hole packs chips at the bottom and risks tap breakage. Q: Can taps and dies be used on stainless steel? Yes, but stainless steel is substantially harder to tap than mild steel and work-hardens quickly. Use a high-speed steel (HSS) or cobalt tap rather than a carbon steel tap. Apply cutting fluid generously — a sulphur-based cutting oil or dedicated tapping compound performs significantly better than general-purpose oils on stainless. Turn the tap forward half a turn, then back a quarter turn to break chips and prevent work-hardening. Use a slower, steadier speed with hand tapping. Q: How do I use a die to cut an external thread? Secure the workpiece vertically in a vice. Apply cutting fluid to the stock. Place the die in the die stock with the chamfered (lead-in) side facing down toward the work. Start by pressing down while rotating slowly — the lead chamfer guides the die squarely onto the stock. Turn forward half a turn, then back a quarter turn to break chips. Keep applying fluid throughout. Check alignment frequently with a square. If resistance builds suddenly, back off and clear chips before continuing. Browse adjustable hand reamers at AIMS Industrial for application support and stock confirmation.
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Tap Types Explained: Taper, Plug, Bottoming, Spiral Point & Flute
Choosing the right tap looks simple until you've snapped one off in a $400 casting. Then you find out the hard way that taper, plug, bottoming, spiral point and spiral flute taps aren't interchangeable — each does a specific job, and using the wrong one in the wrong hole is the fastest route to a broken tool. This guide walks you through every tap type Australian tradies and machinists actually use, with the forum-tested rules for matching the tap to the job. Tap Types — Quick Reference Tap Type Best For Chip Direction Hole Type Taper tap Starting threads by hand Sideways (straight flute) Through or blind (start only) Plug tap General-purpose threading Sideways (straight flute) Through holes Bottoming tap Full threads to base of blind hole Sideways (straight flute) Blind holes (after starter tap) Spiral point (gun) Production tapping in a machine Forward (ahead of tap) Through holes only Spiral flute Blind holes in tough materials Backward (out of hole) Blind holes only Forming (roll) tap Strongest threads, ductile materials No chips (cold-forms) Through or blind The single most common forum mistake: putting a spiral point (gun) tap in a blind hole. The chips have nowhere to go — they pack up at the bottom and snap the tap. Gun taps need somewhere for chips to exit ahead of the cutting flutes. For blind holes, use a spiral flute tap instead. For tap drill sizes, see our Tap Drill Size Chart. For thread standards (BSP, NPT, UNC, UNF, metric), see our Thread Standards Guide. What Is a Tap? A tap is a hardened cutting tool used to create internal threads in a pre-drilled hole. The tap is essentially a precision-ground threaded rod with cutting edges along its length. As you rotate it into a hole sized correctly for the tap, the cutting edges remove material to form the thread profile. Three things define every tap: Thread profile — metric M-series, UNC, UNF, BSW, BSP — must match the thread standard you want to cut. See our Thread Standards Guide for the differences. Chamfer length — the tapered cutting section at the start of the tap. Long chamfer (4–8 threads) for starting, short chamfer (1–2 threads) for cutting threads to the bottom of a blind hole. Flute design — straight, spiral point, or spiral flute. This determines where the chips go as you cut. Match all three to the job and you'll cut clean threads first time. Get one wrong and you'll likely break the tap, strip the thread, or get a hole you can't use. The Three Classical Hand Taps — Taper, Plug, Bottoming Most threading is still done with the traditional set of three straight-flute hand taps. The only difference between them is the length of the chamfered cutting section at the tip — the threads themselves are identical. Taper Tap (4–8 chamfered threads) The taper tap has the longest cutting chamfer — typically 7 to 10 threads of taper at the tip, gradually working up to full thread depth. The long taper does two things: Makes it easy to start the thread square to the hole by hand Distributes the cutting load across many threads, reducing torque and breakage risk Taper taps are the safest tap to start a thread with, especially for tradies tapping by hand without a guide. The trade-off is that you can't cut full threads to the bottom of a blind hole — the tapered tip means the last 7 threads at the bottom are progressively shallower. Plug Tap (3–5 chamfered threads) The plug tap (sometimes called the "second tap") has a shorter chamfer than the taper — usually 3 to 5 threads. It's the workhorse of hand tapping: For through holes, you can start and finish with just a plug tap For blind holes, use it as the second pass after the taper tap to deepen the threads For machine tapping straight-flute work, plug is typically the default If you're only going to buy one hand tap of a given size, make it a plug. Bottoming Tap (1–2 chamfered threads) The bottoming tap has almost no chamfer — usually just 1 to 2 threads of cutting taper at the tip. This lets it cut full threads right down to the bottom of a blind hole. Critical detail tradies often get wrong: a bottoming tap cannot start a thread on its own. With only 1–2 threads of chamfer, it doesn't have the lead-in to stay square in a hole. You must always start with a taper or plug tap first, then finish with the bottoming tap. If you put a bottoming tap into a fresh hole and try to cut threads with it from scratch, it will skate sideways, chip, or break. When to Use Each — Decision Logic Situation Sequence Through hole, easy material Plug tap only Through hole, tough material Taper → plug Blind hole, threads not needed at base Taper → plug Blind hole, full threads to base required Taper → plug → bottoming Hand tapping a sensitive material first time Always start with taper Hand tap sets sold in Australia typically come as a three-piece set (taper, plug, bottoming) in each thread size — see our Imperial Hand Taps and Tap and Die Sets collections. Spiral Point vs Spiral Flute — The #1 Confusion This is the question that breaks more taps than any other: spiral point and spiral flute taps look almost identical, and tradies who haven't worked with both can't tell them apart in a tool drawer. They are designed for opposite jobs. Get them mixed up and you'll snap a tap inside a workpiece. Spiral Point Tap (also called Gun Tap, Bull Nose Tap) A spiral point tap has straight flutes along its length, but the cutting chamfer at the tip is ground with a slight angled "point" geometry that pushes chips forward, ahead of the cutting edge. The chips clear out through the bottom of the hole. This makes spiral point taps brilliant for: Through holes — chips exit the bottom of the hole, never building up around the tap Production machine tapping — high speed, continuous rotation, no need to back off Soft and stringy materials like aluminium, brass and copper where chips tend to weld to a cutting tap Spiral point taps are also called "gun taps" because the chip-shooting action looks like a rifle barrel ejecting cartridges. Critical warning: NEVER use a spiral point tap in a blind hole. The chips have nowhere to go. They pack up at the bottom of the hole, the tap binds, and it snaps. This is the most common forum-reported broken-tap scenario. See our Metric Spiral Point Taps and Imperial Spiral Point Taps ranges. Spiral Flute Tap A spiral flute tap has helical flutes running along its length, similar to a drill bit. The helix direction pulls chips backward, out of the hole as the tap rotates. This makes spiral flute taps the right choice for: Blind holes — the upward chip flow keeps the bottom of the hole clear Tough materials like stainless steel, alloy steel, and titanium where good chip evacuation prevents work-hardening Deep threading where chips have a long way to travel before exiting Use a spiral flute tap in a through hole and you'll get a worse result than a plug — the helical chip flow pulls chips back through the cutting threads, sometimes re-cutting them. Spiral flute should mostly stay in blind-hole work. See our Metric Spiral Flute Taps and Imperial Spiral Flute Taps. How to Tell Spiral Point and Spiral Flute Apart Feature Spiral Point (Gun) Spiral Flute Flutes along body Straight (parallel to axis) Helical (twisted like a drill) Chamfer geometry Angled "point" at tip Standard cutting chamfer Chip direction Forward (out bottom) Backward (out top) Use in through holes ✓ Excellent ✗ Avoid Use in blind holes ✗ Will snap ✓ Excellent The visual giveaway is the flute geometry — if the flutes are straight (parallel to the tap axis), it's spiral point. If the flutes spiral around the tap like a drill bit, it's spiral flute. The chamfer geometry difference is harder to see without comparing two side by side. Hand Taps vs Machine Taps Hand taps and machine taps look similar but are engineered for different working conditions. Hand Taps Hand taps are designed for manual use with a tap wrench. They typically come as a three-piece set (taper, plug, bottoming) and have straight flutes for general-purpose threading. The expectation is that the user will rotate the tap forward 1/2 to 1 turn, then back off 1/4 to 1/2 turn to break the chip — this is essential for chip clearance with straight-flute geometry. Hand taps work fine in machine spindles too, especially for small-batch work. But for production tapping, machine taps cut faster and cleaner. Machine Taps Machine taps (also called CNC taps) are designed for continuous high-speed rotation in a tapping head, drill press, lathe, or CNC machine. They're typically: Spiral point or spiral flute (so they don't need backing off to break chips) Made from tougher materials (HSS-E cobalt, HSS-PM, or carbide) Often coated for higher cutting speeds Ground to tighter tolerances for repeatable thread quality If you're tapping the same thread hundreds of times a day, a machine tap pays for itself in cycle time and tool life. For one-off jobs or maintenance work, a hand tap set is more economical. Forming Taps (Roll Taps) — Cold-Forming Threads Forming taps (also called roll taps, fluteless taps or thread-rolling taps) are a fundamentally different way to make a thread. Instead of cutting and removing material, a forming tap displaces the workpiece material — it cold-forms the thread profile by pressing the metal into the thread shape. Forming taps have no flutes (no chip path is needed because there are no chips), no cutting edges, and a smooth polygonal cross-section that does the forming work. Advantages of forming taps: Stronger threads — the cold-worked metal grain follows the thread profile rather than being cut across it, increasing thread strength by 10–40% No chips — eliminates chip evacuation problems and chip welding in soft materials Longer tool life — no cutting edges to dull or break Higher feed rates — typically 30–50% faster than cutting taps Smaller tap drill — forming taps use a slightly larger pilot hole than cutting taps Strict limits — forming taps DON'T work on: Cast iron (brittle — will crack instead of forming) Hardened steel above ~30 HRC Most plastics (cold flow won't hold thread shape) Materials with less than ~5% elongation Forming taps are best suited to aluminium alloys, low-to-medium carbon steel, copper, brass (soft), and other ductile metals. See Metric Thread Forming Taps for our range. Tap Materials — HSS, HSS-E, HSS-PM, Solid Carbide The material the tap itself is made from determines its hardness, toughness, heat resistance, and price. Match the tap material to the workpiece material and the production rate. HSS (High-Speed Steel) HSS is the workhorse tap material. Tough, takes a sharp edge, holds up well to general-purpose threading in mild steel, aluminium, brass, plastics, and most workshop materials. M2 HSS is the most common grade. Affordable enough that breaking one isn't a disaster. Limitation: HSS softens at sustained cutting temperatures above ~600°C. Not the right pick for high-speed production work or work-hardening stainless steel. HSS-E (HSS-Co, Cobalt HSS) HSS-E adds 5–8% cobalt to HSS, increasing hot hardness — the ability to retain cutting hardness at high temperatures. This makes HSS-E better suited to: Stainless steel (which work-hardens and generates heat) Heat-resistant alloys Higher cutting speeds where HSS would soften Production tapping where consistent edge life matters HSS-E is typically 20–40% more expensive than plain HSS but lasts considerably longer in tough materials. HSS-PM (Powder Metallurgy HSS) HSS-PM is made by powder metallurgy rather than melted-and-rolled steel. The fine grain structure gives it better toughness than HSS-E with similar or higher hardness. Use cases: Very hard alloys Tool steel and die work High-precision tapping where edge consistency matters Bridge between cobalt HSS and solid carbide Premium price, but still tougher than carbide — won't shatter if you stall it. Solid Carbide Solid carbide taps are extremely hard and wear-resistant, designed for high-volume CNC tapping in difficult materials. They allow much higher cutting speeds than any HSS variant. The trade-off is brittleness — carbide will shatter if you stall it, side-load it, or hit a hard inclusion. Carbide taps need a rigid setup, precision-controlled feed, and a tap holder with sufficient tension/compression compensation. Not for hand tapping. Tap Coatings — Black Oxide, TiN, TiCN, TiAlN Surface coatings reduce friction, improve chip flow, and extend tap life. Common coatings you'll see on Australian shelves: Black oxide / steam tempered — a thin oxide layer that reduces galling in mild steel and improves lubricant retention. Cheap and reliable for general-purpose work. TiN (Titanium Nitride) — gold-coloured coating, ~2,300 HV hardness, good general-purpose coating that extends tap life in steel and stainless. TiCN (Titanium Carbonitride) — blue-grey or grey-purple, harder than TiN (~3,000 HV), good for cast iron and harder steels. TiAlN (Titanium Aluminium Nitride) — violet-grey or dark grey, very hard (~3,300 HV), excellent heat resistance — the premium choice for stainless, high-temp alloys, and hardened materials at high cutting speeds. For most workshop tapping, an uncoated or black-oxide HSS tap with proper cutting fluid is fine. Move up to TiN or TiAlN when you're tapping stainless, working at production rates, or running unattended CNC tapping cycles. How to Choose a Tap by Workpiece Material Material is the single biggest factor in tap selection. Get the material match right and you'll cut clean threads with a tap that lasts. Aluminium and Aluminium Alloys Aluminium tapping has one big problem: the chips weld to a hot HSS cutting tap and clog the flutes — known as "BUE" (built-up edge) on machinist forums. The result is a torn-looking thread with chunks of aluminium stuck to it. Fixes that work: Use a spiral point (gun) tap at higher cutting speed — gets chips out before they weld Use a forming tap — no chips at all, beautiful threads, ideal for aluminium Lubricate generously with kerosene, methylated spirits, or a dedicated aluminium-tapping fluid (see our Tap Magic cutting fluid guide for variant selection) — water-soluble coolants alone aren't enough Don't dwell — keep the tap moving so chips don't weld Mild Steel The easiest material to tap. HSS plug tap or spiral point (through holes), spiral flute (blind holes). General-purpose cutting fluid. Hand tapping or machine tapping both work well. This is what most workshop tapping looks like. Stainless Steel Stainless is the material that breaks more taps than any other — and forums are full of advice about it because most of that advice is wrong. The key facts: Stainless work-hardens. If the tap "rides" the surface without cutting (because it's dull, you're going too slow, or there's no cutting fluid), the surface hardens and the next tap breaks trying to cut into it. Use HSS-E (cobalt) or HSS-PM taps, not plain HSS. The hot hardness matters here. Use sulfurised cutting oil ("dark cutting oil") — water-based coolants alone aren't enough. The sulfur prevents chip welding. Spiral flute for blind holes, spiral point for through. Don't compromise here. Cutting speed: slower than you think. Roughly 1/3 to 1/2 the speed you'd use for mild steel. Engage the cutting edge immediately. Never let the tap idle on the surface. Cast Iron Cast iron tapping is the opposite of stainless — it cuts easily but throws abrasive dust instead of chips. Use: Straight-flute or spiral point HSS or HSS-E taps NO cutting fluid (cast iron is self-lubricating; fluid just turns dust to paste) Compressed air to clear chip dust Never use forming taps on cast iron — it's brittle and will crack instead of forming Brass and Bronze Brass is forgiving — almost anything works. Standard HSS plug tap, light cutting oil, moderate speed. Bronze is similar but harder, so prefer HSS-E for production work. Watch out for "gummy" brass alloys that grab the tap — back off frequently to break chips. Plastic For most plastics (acrylic, nylon, polycarbonate, ABS), use a sharp HSS tap with shallow chamfer, slow speed, and dry cutting. No forming taps (plastic creeps and won't hold a formed thread). For abrasive filled plastics (glass-filled nylon, carbon-filled), use a TiN-coated tap for better wear life. Why Taps Break — and How to Avoid It "Why do my taps keep breaking?" is the #1 forum question in tapping. The answer is almost always one of these: Wrong tap for the hole type. Spiral point in a blind hole = packed chips = broken tap. Bottoming tap as a starter = skating tap = broken tap. Wrong tap drill size. Too small a hole means the tap has to cut too much material per thread. Always verify against a Tap Drill Size Chart before drilling. No cutting fluid. Friction heat softens the tap, chips weld, breakage follows. Even on "easy" materials. Not backing off for chip break. Straight-flute hand taps need a 1/4 to 1/2 reverse turn for every 1 to 2 forward turns to break chips. Forget this and the chips pack up. Wrong material match. Plain HSS in stainless = work-hardened surface = broken tap on the next try. Use HSS-E or HSS-PM. Tap not square to the hole. Side-loading a tap as it cuts puts a bending stress that taps don't handle well. Use a tapping guide or set up in a machine. Forcing through resistance. If a tap suddenly gets harder to turn, STOP. Back it out, clear chips, check for binding, and continue. Don't crank harder. Riding the surface without cutting (stainless especially) — work-hardens the material and breaks the next tap. If you do break a tap, see our Screw Extractors for tap removal tools. Alternatives: EDM tap disintegration (specialist service), or carbide-burr the broken tap out (last resort, damages the thread). Tap Speed and Cutting Fluid Cutting speed for tapping is dramatically slower than for drilling. Approximate starting speeds for HSS hand taps: Material SFM Approx RPM for M10 Aluminium 50–100 500–1000 Brass / bronze 50–100 500–1000 Mild steel 30–60 300–600 Cast iron 20–40 200–400 Stainless steel 10–20 100–200 Tool steel (annealed) 10–20 100–200 HSS-E and HSS-PM taps can run 50–100% faster than these numbers. Solid carbide can run faster still, but only in rigid machine setups. Cutting fluid pairings: Mild steel, cast steel: standard tapping fluid or sulfurised cutting oil Stainless steel, alloy steel: sulfurised cutting oil ("dark oil") Aluminium: kerosene, methylated spirits, or dedicated aluminium-cutting fluid Brass, bronze: light mineral oil or dry Cast iron: dry (compressed air for chip removal) Plastic: dry, or water-based mist for filled plastics See Cutting Lubricants for our range. Sutton Tools — Australian-Made Cutting Tools Sutton Tools is Australia's largest manufacturer of HSS cutting tools, based in Thomastown, Victoria, and Australian-owned and operated since 1917. Their tap range is genuinely made in Australia (not just badged) — Sutton manufactures everything from the steel grinding through to coating in their Thomastown plant. The Sutton tap range covers: Premium HSS Blue series — bright, cobalt-tough, suited to mild steel, alloy steel and stainless Premium HSS Ni — for nickel alloys and tough stainless grades Tinite coated — TiN-coated for extended life R45 series — proprietary geometries for difficult materials (W, Al, VADH variants for steel, aluminium and high-strength materials) Spiral flute, spiral point and straight flute in metric (M3 to M30) and imperial (UNC, UNF, BSW, BSP) Sutton taps carry the Australian Made & Owned certification. For Australian Industry Capability (AIC), Buy Australian, mining local content and government procurement requirements, Sutton ticks every box. See our Sutton Tools collection or browse all our Taps. AIMS Tap Product Cross-Reference Sourcing taps and threading tools from AIMS Industrial by category: Hand taps (metric & imperial): Imperial Hand Taps Spiral point (gun) taps — metric: Metric Spiral Point Taps Spiral point (gun) taps — imperial: Imperial Spiral Point Taps Spiral flute taps — metric: Metric Spiral Flute Taps Spiral flute taps — imperial: Imperial Spiral Flute Taps Straight flute taps — metric: Metric Straight Flute Taps Straight flute taps — imperial: Imperial Straight Flute Taps Thread forming (roll) taps: Metric Thread Forming Taps Machine nut taps: Metric Machine Nut Taps | Imperial Machine Nut Taps Complete tap and die sets: Tap and Die Sets Thread identification gauges: Screw Pitch Gauges Tap extractors (broken tap removal): Screw Extractors Cutting fluids: Cutting Lubricants Full threading range: Threading Collection Related Reference Articles Tap Drill Size Chart — Metric & Imperial (drill size for every tap) Thread Standards Guide — BSP vs NPT vs UNC Tap Drill Diameters Explained — Major, Minor & Pitch Drill Bit Size Chart — Metric, Imperial, Fractional Repairing stripped threads rather than starting fresh? Our Stripped Threads Repair Guide walks the escalation ladder — re-tap, oversize, Helicoil, TimeSert and weld-up. Frequently Asked Questions What is the difference between a taper, plug and bottoming tap? The difference is the length of the chamfered cutting section at the tip. Taper taps have 7–10 chamfered threads (longest, easiest to start). Plug taps have 3–5 chamfered threads (most common general-purpose tap). Bottoming taps have only 1–2 chamfered threads, allowing them to cut full threads to the bottom of a blind hole — but they can't start a thread on their own; you must use a taper or plug first. What is the difference between a spiral point and a spiral flute tap? A spiral point (gun) tap pushes chips forward, ahead of the tap — use it in THROUGH holes only. A spiral flute tap pulls chips backward, up out of the hole — use it in BLIND holes only. The flute geometry on the tap body is the visual giveaway: straight along the axis = spiral point; helical like a drill bit = spiral flute. Using one in the wrong hole type is the #1 cause of broken taps. What is a gun tap? "Gun tap" is the common workshop name for a spiral point tap. The name comes from the way it ejects chips forward, ahead of the cutting edge, like a gun ejecting cartridges. Gun taps are ideal for through-hole production machine tapping. Never use them in blind holes — the chips will pack and snap the tap. Can a bottoming tap start a thread? No. A bottoming tap has only 1–2 threads of chamfer, which isn't enough lead-in to keep the tap square in a fresh hole. It will skate sideways, chip, or break. Always start the thread with a taper or plug tap first, then use the bottoming tap to finish the thread to the base of a blind hole. What tap should I use for aluminium? A spiral point (gun) tap for through holes, or a forming (roll) tap for either through or blind. Aluminium chips weld to a standard cutting tap as it heats up — gun taps clear chips fast enough to avoid this, and forming taps produce no chips at all. Use kerosene, methylated spirits, or a dedicated aluminium tapping fluid as lubricant. Never tap aluminium dry. What tap should I use for stainless steel? An HSS-E (cobalt HSS) or HSS-PM tap, ideally with TiAlN coating. Spiral flute for blind holes, spiral point for through. Use sulfurised cutting oil (dark cutting oil) — not water-based coolant alone. Cut at 1/3 to 1/2 the speed you'd use for mild steel, and never let the tap "ride" on the surface without cutting, or the stainless will work-harden and break your next tap. Why do my taps keep breaking? The most common causes are: spiral point tap in a blind hole (chips pack), wrong tap drill size (hole too small), no cutting fluid, not backing off chips with straight-flute hand taps, wrong material match (plain HSS in stainless), tap not square to the hole, or forcing through resistance. Stop the moment a tap suddenly gets harder to turn — clear chips, check alignment, and continue. Hand tap or machine tap — which do I need? For one-off jobs, maintenance work, and small production runs, hand taps (taper, plug, bottoming three-piece sets) used with a tap wrench or in a drill press are fine. For production tapping at high volume, machine taps (typically spiral point or spiral flute, HSS-E or coated) cut faster and don't require backing off. Hand taps work in machines too, just slower. What is a forming tap (roll tap)? A forming tap cold-forms the thread by displacing material — no cutting, no chips. The resulting thread is 10–40% stronger than a cut thread because the metal grain follows the thread profile. Forming taps work on ductile materials (aluminium, low-carbon steel, copper) but cannot be used on cast iron, hardened steel, brittle materials, or most plastics. They require a slightly larger pilot hole than cutting taps. What does HSS-E mean? HSS-E is high-speed steel alloyed with 5–8% cobalt (also called HSS-Co or "cobalt HSS"). The cobalt addition increases hot hardness — the tool's ability to retain hardness at elevated cutting temperatures. HSS-E taps last considerably longer than plain HSS in stainless steel, alloy steel and at higher cutting speeds. What is HSS-PM tap material? HSS-PM (Powder Metallurgy HSS) is high-speed steel made by powder metallurgy rather than melted-and-rolled steel. The fine, even grain structure gives it better toughness than cobalt HSS with similar or higher hardness. It sits between HSS-E and solid carbide in price and performance — used for tough materials, tool steel, and precision tapping where consistent edge life matters. What is the difference between a tap and a die? A tap cuts internal threads (in a hole). A die cuts external threads (on a rod or pipe). Tap-and-die sets pair both tools at matching thread sizes so you can create or repair both sides of a threaded joint. See our Tap and Die Sets for combined kits. Do I need cutting fluid when tapping? Yes — for almost every material except cast iron. Even on mild steel where the tap feels easy, cutting fluid extends tap life dramatically, improves thread surface finish, reduces breakage risk, and prevents chip welding. Cast iron is the exception — its graphite content makes it self-lubricating, and adding fluid turns the chip dust to abrasive paste. How fast should I tap? Tapping speed is much slower than drilling. As a starting point with HSS taps: aluminium and brass 50–100 SFM, mild steel 30–60 SFM, cast iron 20–40 SFM, stainless steel 10–20 SFM. HSS-E (cobalt) taps can run 50–100% faster. If a tap chatters, screams, or smokes, slow down and add cutting fluid. What does the "chamfer" of a tap mean? The chamfer is the tapered cutting section at the tip of the tap, where the cutting edges aren't yet at full thread depth. The chamfer length is what distinguishes taper taps (7–10 chamfered threads), plug taps (3–5), and bottoming taps (1–2). A longer chamfer is easier to start but can't cut threads near the bottom of a blind hole. People Also Ask — Tap Types Q: What are the three classical hand tap types and how do they differ? The three classical hand taps are taper, plug, and bottoming. A taper tap has the most chamfered lead threads, making it easiest to start but unable to cut to the bottom of a blind hole. A plug tap has fewer lead threads and is the most common general-purpose choice. A bottoming tap has just one or two lead threads and cuts threads to the very base of a blind hole. Q: What is the difference between a spiral point tap and a spiral flute tap? A spiral point (gun) tap pushes chips forward ahead of the cut and is used in through holes where chips can exit from the far end. A spiral flute tap pulls chips back up and out, suited to blind holes where chips cannot be pushed through. Using the wrong type in a blind hole is one of the most common causes of tap breakage. Q: What is a forming (roll) tap and when would you use it? A forming or roll tap displaces material to create threads rather than cutting them, producing no chips. This gives stronger threads and is well suited to ductile materials such as aluminium and low-carbon steel. Forming taps cannot be used in brittle materials, which will crack rather than form. Q: When should you use a machine tap rather than a hand tap? Machine taps are designed to run in a power tapping head, CNC machine, or cordless drill at controlled speed and feed. Hand taps are designed for manual use where the operator can feel resistance and reverse to break chips. Using a hand tap in a power machine without controlled feed typically results in tap breakage. Q: Why is starting with a taper tap important for hand tapping? The chamfered lead on a taper tap helps align the tap square to the hole before the cutting threads engage. Starting square is critical — a tap driven at an angle cuts a crooked thread and is prone to breakage, particularly in blind holes and harder materials. AIMS Industrial stocks machining — see the full range for trade and industrial use. Use the AIMS Tap & Drill Selector tool to match the right tap and drill to your thread type.
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