Thread 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.

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

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

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

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

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

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.

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

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

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.

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.

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:

1. 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.
2. 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).
3. Check the thread angle with a thread angle gauge. 55° = British heritage (BSP, BSW). 60° = American or metric (NPT, UNC, UNF, M).
4. Measure the pitch with a thread pitch gauge. Compare against the size tables above to confirm the standard.
5. 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:

1. 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.
2. 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.
3. 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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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

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.

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