Thread identification
BSPP — British Parallel (G thread)
BSPP ('G') threads are parallel Whitworth-form threads that don't seal on the threads at all — the seal is a 60° cone seat, a bonded washer, or an O-ring. They're the default on British, much European, and older Japanese equipment.
Identify it
Four checks at the bench.
- 01
Confirm a parallel (non-tapered) thread with a 55° Whitworth form
- 02
Female hose ends usually carry a 60° cone seat inside the nose
- 03
Measure OD and match to BSP nominal sizes (a G1/4 measures about 0.52" OD)
- 04
Port fittings seal with a bonded washer or O-ring at the shoulder
Don't confuse it with…
- NPT sizes measure close at a glance; pitch and flank angle differ — the classic imported-equipment leak
- JIC's 37° flare vs BSPP's 60° cone: both look like cones, seal angles differ
Reference
Size chart.
| Size | Thread |
|---|---|
| G1/8 | 28 TPI |
| G1/4 | 19 TPI |
| G3/8 | 19 TPI |
| G1/2 | 14 TPI |
| G3/4 | 14 TPI |
| G1 | 11 TPI |
A1 stocks it.
- BSPP Female Swivel 60° ConeA1-WB-BSPF
- Interlock BSPP Female 60° ConeA1-4S-BSPF
- BSP ↔ NPT Adapter SeriesA1-FA-BNPT
Unsure even after the checks? Bring the fitting to the counter — gauging an unknown thread takes us about a minute. (817) 589-7101.
BSPP — British Parallel (G thread) — FAQs
What does the G in G1/2 mean?
It marks a parallel BSP thread (from the German 'Gewinde'). R1/2 marks the tapered version, BSPT.
How do BSPP fittings seal if the thread is parallel?
On a 60° cone in the swivel nose, or at ports with a bonded washer or O-ring under the shoulder — never on the threads.
My import machine leaks at every fitting after a US rebuild — why?
Someone likely installed NPT parts into BSPP ports. They start smoothly and never seal; we stock the correct BSPP parts and conversion adapters.