A thread that won't match anything in your box of fittings is almost always a different standard — not a damaged one. Three measurements tell you which, and we can cut or copy any of them.
Threads look interchangeable until they aren't. The reason a fitting "almost" threads in is usually that you're mixing standards — a metric thread into a UNF hole, or a BSP fitting into an NPT port. To identify any external thread you only need three numbers:
Measure the outside diameter of the thread crests with calipers (for a hole, measure the minor diameter).
Use a thread pitch gauge — the leaf that seats with no rocking gives metric pitch in mm or imperial threads-per-inch.
60° for metric & Unified, 55° for BSP/Whitworth. A visibly tapered thread is a pipe thread (NPT/BSPT).
| Family | Angle | Pitch stated as | How to spot it |
|---|---|---|---|
| Metric (M) | 60° | mm (e.g. 1.5) | Pitch gauge reads a round mm value |
| UNC / UNF | 60° | TPI (e.g. 20) | Pitch gauge reads whole threads-per-inch |
| BSW / BSF | 55° | TPI | 55° leaf seats; rounded crests |
| BSP (G / R) | 55° | TPI | Pipe thread, 55°; common outside North America |
| NPT / NPTF | 60° | TPI | Tapered pipe thread, 60°; North American |
| ACME / trapezoidal | 29° / 30° | per inch / mm | Square-ish lead-screw profile |
A frequent trap: 1/8 NPT vs 1/8 BSP look almost identical but have different angles and pitches, so a fitting will start, cross-thread, and leak. Confirm the angle before you seal anything.
Identifying a thread is useful if you want to buy a standard fitting. But if the threaded part is bespoke or discontinued, naming the standard is optional — we measure the thread directly off your sample and cut a matching one. We machine metric, UNC/UNF, BSW/BSF, BSP/BSPT, NPT/NPTF, ACME and fully custom threads, internal or external.
Send the mating part if you have it. The fitting or port the thread screws into is the best gauge of all — it confirms size, pitch and class in one go. No drawing? Use no-CAD ordering, or see how we handle threaded components in reverse engineering.
Internal threads are harder because you can't lay a caliper across the crests. Three practical methods, in order of accuracy:
If none of that is conclusive, it's the same answer as everything else on this page: send us the part. We gauge the internal thread directly and cut a mating component to fit it — no name required.
Measure major diameter (calipers), pitch (pitch gauge), and thread angle (60° metric/Unified, 55° BSP/Whitworth). Add whether it's tapered to separate pipe threads. Those values place almost any thread.
Both are 60°, so check pitch: metric is mm between threads, imperial is threads-per-inch. The pitch-gauge leaf that seats without rocking gives the system and value.
BSP is 55°, NPT is 60°, and their pitches differ at the same nominal size. BSP is common outside North America, NPT inside it. They don't interchange — identify before sealing.
Yes. Send the part; we measure the thread on the sample and cut a match. We machine metric, UNC/UNF, BSW/BSF, BSP/BSPT, NPT/NPTF, ACME and custom threads.
Send the threaded part (and its mate if possible). Engineers reply within 24 hours.
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