BSPP vs BSPT Threads Explained

Almost every air fitting in Ireland uses a BSP (British Standard Pipe) thread. There are two types, and getting them mixed up is the most common cause of leaks we see.

BSPP: parallel

BSPP threads stay the same diameter along their length. They are marked with a G, for example G1/4". A parallel thread does not seal on the thread itself, so it needs a bonded washer, O-ring or sealing face.

BSPT: tapered

BSPT threads get slightly narrower towards the end. Male tapered threads are marked R, for example R1/4". The taper wedges tight as you screw it in, and with PTFE tape or thread sealant it seals on the thread.

How to tell them apart

• Look along the thread. If it narrows towards the tip, it is tapered.
• Check the marking or part number. G means parallel, R means tapered.
• If a fitting has a sealing washer or O-ring, it is almost always parallel.

Can I mix them?

A tapered male (R) into a parallel female (Rp) is common and seals well with tape or sealant. A parallel male into a tapered female will not seal properly and should be avoided.

Size chart

BSP sizes do not match the measured diameter, which catches a lot of people out. Approximate outside diameter of the male thread:

• 1/8": 9.7mm
• 1/4": 13.2mm
• 3/8": 16.7mm
• 1/2": 21.0mm
• 3/4": 26.4mm
• 1": 33.2mm

Bring in the old fitting and we will match it for you at our Little Island counter, or call 021 435 5424. Browse adaptors and hose tails.