British BSP Threads: BSPP, BSPT, and Hydraulic Sealing
HGW Hydraulics on Mar 8th 2022
British Standard Pipe threads appear on many hydraulic and fluid power components. The two common forms are BSPP, which is British Standard Pipe Parallel, and BSPT, which is British Standard Pipe Tapered.
These thread forms are historically related to Whitworth thread standards. They can be confused with NPT or NPSM connections because the sizes may look close, but the pitch, thread form, and thread angle are not the same.
BSPP Parallel Threads
A BSPP male thread mates with a BSPP female fitting or a female port connection. The threads are straight, and many BSPP hydraulic connections use a 30 degree seat or an O-ring or soft metal sealing element to complete the seal.
The thread angle is 55 degrees. That is different from the 60 degree angle used on NPSM-style straight mechanical pipe threads, so BSPP should not be treated as interchangeable with NPSM.
BSPT Tapered Threads
A BSPT male thread is tapered. It can mate with a BSPT female thread, and in some cases it may be used with a BSPP parallel female thread. In these connections, the seal is made on the threads.
Like BSPP, BSPT uses a 55 degree thread angle. It should not be mixed with NPTF or other 60 degree tapered pipe threads simply because the nominal size appears similar.
G and R Size Markings
British thread sizes are often shown as fractional dimensions with a letter before the size. A G marking normally identifies a parallel BSPP thread. An R marking normally identifies a tapered BSPT thread.
For example, a BSPT size may be written with a fractional size and thread count. That notation should be checked together with measured outside diameter, pitch, thread angle, and sealing style.
British Flange Compatibility
British thread standards are not interchangeable with NPT-style fittings, but flange fittings in British-standard systems may align with familiar SAE Code 61 or Code 62 flange patterns. A technician should separate thread identification from flange identification.
When replacing a British hydraulic connection, confirm BSPP versus BSPT, seat or O-ring style, thread angle, pitch, and mating port before choosing the adapter.