Product Guides

Tap & 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.