Thread identification
BSPT — British Tapered (R thread)
BSPT ('R') is the tapered British pipe thread, sealing on thread deformation like NPT but with a 55° flank angle and Whitworth form. It appears on older British and Asian equipment ports and will cross-start with NPT — then leak.
Identify it
Four checks at the bench.
- 01
Confirm taper along the thread length
- 02
Check flank angle: 55° gauge fits cleanly, 60° doesn't
- 03
Compare pitch: BSPT 1/2" is 14 TPI like NPT, but 3/8" is 19 TPI vs NPT's 18 — one thread per inch says everything
- 04
Look for 'R' or 'PT' markings on OEM plumbing
Don't confuse it with…
- NPT at 60° — the single most common thread mix-up we see at the counter
- 'PT' markings on Japanese equipment are BSPT, not NPT
Reference
Size chart.
| Size | Thread |
|---|---|
| R1/8 | 28 TPI |
| R1/4 | 19 TPI |
| R3/8 | 19 TPI |
| R1/2 | 14 TPI |
| R3/4 | 14 TPI |
A1 stocks it.
Unsure even after the checks? Bring the fitting to the counter — gauging an unknown thread takes us about a minute. (817) 589-7101.
BSPT — British Tapered (R thread) — FAQs
Can I put an NPT fitting in a BSPT port?
It will thread in partway and may even hold briefly — then weep under pressure and heat cycles. Use the correct thread or a proper conversion adapter.
How can I tell BSPT from NPT quickly?
Pitch gauge: at 3/8", BSPT is 19 TPI, NPT is 18. At 1/2" both are 14 TPI, so check flank angle (55° vs 60°) with a profile gauge.