NPT vs BSPT vs G Pipe Threads

NPT, BSPT and G are all pipe threads, but they are not interchangeable. NPT has a 60° thread angle, while BSPT and G have a 55° thread angle. NPT and BSPT are tapered, and G is parallel.

NPT

NPT (National Pipe Taper) is the American tapered pipe thread, covered by ANSI/ASME B1.20.1. It seals by the taper fit and is checked with a basic or min/max step gauge.

BSPT

BSPT is the British tapered pipe thread, covered by BS 21 and ISO 7-2, where it is called ISO 7-R. It also seals by the taper.

G (BSPP)

G threads are parallel and covered by ISO 228-2. The seal is made outside the thread, for example with a washer or gasket, so the gauge checks the thread fit only.

Why the gauge has to match

Because the angle and form differ, a gauge for one pipe thread cannot be used to check another, even when the nominal size looks similar. Check the standard on your drawing and choose the gauge to match.