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Identical nominal size, different thread
| Nominal | NPT TPI | BSPT TPI | Flank angle | Seal method |
|---|---|---|---|---|
| 1/8" | 27 | 28 | NPT 60° / BSP 55° | taper wedge |
| 1/4" | 18 | 19 | same | taper wedge |
| 3/8" | 18 | 19 | same | taper wedge |
| 1/2" | 14 | 14 | same | taper wedge |
| 3/4" | 14 | 14 | same | taper wedge |
Note: same TPI at 1/2" and 3/4" is exactly why people assume they match. They do not — the flank angle differs.
Field identification
- Count TPI with a thread gauge.
- Check the flank: pointed (60°) = NPT; rounded (55°) = BSP.
- If unsure, thread a known NPT nipple in by hand — binds after one turn = BSP.
BSPP note
Parallel BSPP seals on a bonded washer / O-ring at the face, not the thread. Never apply NPT "tape-the-whole-thread" logic to BSPP.
How to cite: JIAJIE Pneumatic, "BSP vs NPT Thread Size Guide", xingjiali.cn/resources/thread-size-guide.html
Companion product line: Stainless Steel Precision Fittings · Pneumatic Quick Couplers