An alternative to tapered threads in pipe joints is the use of parallel threads, similar to the threads of machine screws and bolts. Since parallel threads are incapable of forming a pressure-tight seal on their own, the sealing action of a parallel thread pipe ﬁtting must be achieved some other way. This function is usually met with an O-ring or gasket.
In the United States, a common design of parallel-thread pipe ﬁtting is the SAE straight thread, named after the Society of Automotive Engineers:
Sealing is accomplished as the O-ring is compressed against the shoulder of the female ﬁtting. The threads serve only to provide force (not ﬂuid sealing), much like the threads of a fastener. Another parallel-thread pipe standard is the BSPP, or British Standard Pipe Parallel. Like the BSPT (tapered) standard, the thread angle of BSPP is 55 o . Like the SAE parallel-thread standard, sealing is accomplished by means of an O-ring which compresses against the shoulder of the matching female ﬁtting: