1/2″ NPT vs 1/2″ BSPT
Same nominal size, two different standards. Side by side at this size only.
1/2″ NPT and 1/2″ BSPT both have 14 threads per inch. This is the worst case: the pitch matches, so the two engage smoothly and feel completely normal in the hand. Nothing warns you. The joint then weeps under pressure because the flanks are cut at different angles — 60° against 55° — and only touch at their tips.
The numbers
| 1/2″ NPT | 1/2″ BSPT | Difference | |
|---|---|---|---|
| Threads per inch | 14 | 14 | identical |
| Thread angle | 60° | 55° | 5° |
| Thread form | Flat crest & root | Rounded (Whitworth) | — |
| Major diameter | 0.84″ | 0.825″ | 0.015″ |
| Major diameter (mm) | 21.34 | 20.95 | 0.39 |
| Pitch diameter at gauge plane | 0.76843″ | 0.7842″ | 0.0158″ |
| Taper | 1:16 | 1:16 | identical |
| Tap drill | 23/32 | 18.6 mm | — |
| Hand-tight engagement | 0.32″ (~4.5 turns) | 0.291″ (~4.1 turns) | — |
| Standard | ASME B1.20.1 | ISO 7-1 / BS 21 | — |
Telling them apart before you assemble
Both threads are tapered at the same 1 in 16, so taper is no help here. Two things are:
Count the threads over one inch. Useless at 1/2″ — both are 14 TPI. Skip to the next test.
Look at the crest of the thread under magnification. NPT crests are flat, cut off square. BSPT crests are rounded. This works at every size and is the test that never lies, but it needs a loupe and decent light.
What to use instead
A 1/2″ NPT to 1/2″ BSPT adapter, with the correct thread cut on each end. They are inexpensive and standard stock. Do not attempt to make one thread work in the other by adding more PTFE tape — tape fills the spiral leak path along a correctly matched thread, it does not compensate for a five degree flank mismatch.