Home / Guides / BSPP or BSPT: is my BSP thread parallel or tapered?
BSPP or BSPT: is my BSP thread parallel or tapered?
BSP comes in two forms, parallel G and tapered R, and they seal in completely different ways.
One family, two sealing methods
BSP, the British Standard Pipe thread, splits into parallel and tapered. Parallel BSP is written BSPP or G and follows ISO 228; its flanks run straight so the thread cannot seal by itself, and it relies on a washer or O-ring at the face. Tapered BSP is written BSPT or R; its diameter narrows along the thread so the flanks wedge and seal on the thread.
Both share the same thread form and pitch for a given size, which is why a G and an R part of the same size will often start together even though they seal differently.
How to tell them apart
Look along the thread. A parallel thread keeps the same diameter from end to end. A tapered thread visibly gets narrower toward the tip. Roll a straightedge along the crests if you are unsure: parallel stays flush, tapered pulls away.
A female parallel fitting usually has a flat seat or a washer face inside, while a tapered joint has no washer and depends on thread engagement, often with sealant.
Sealing each correctly
Seal a parallel BSPP joint with a fresh washer or a bonded seal at the face. Do not pile on PTFE tape to make a parallel thread grip, because the seal is the washer, not the thread. Seal a tapered BSPT joint with PTFE tape or pipe sealant on the thread, wrapped in the direction of tightening, because there the thread is the seal.
Mixing a parallel male into a tapered female, or the reverse, can start and even feel tight while still weeping, so match G to G and R to R where you can.
Common tap sizes
On outdoor taps the usual sizes are 1/2 inch BSP and 3/4 inch BSP. Remember the nominal size is not the outer diameter: 3/4 inch BSP measures around 26.4mm across the thread. Identify the size and the form together, then buy the matching washer or sealant for the seal type.
Get the right fitting
Open the HoseFitFinder toolMatch parallel or tapered BSPFind BSP parallel to tapered adapter on eBayaffiliateRelated
Sources: ISO 228 (BSP) thread standard