HOW TO ID BRITISH (BSP) THREADS
British Pipe Standard fittings are amongst the most popular of all foreign threads in the world today. These threads comes in the two versions : Parallel (BSPP) and Tapered (BSPT).
The threads flank angle for both tapered & parallel British threads in 55O. It is a fairly common mistake to identify a BSPT (tapered) as an NPT. Always remember that NPT threads have a 60° thread flank angle, and BSPT has a 55° angle (this can be verified with a thread gauge). Although BSP is a foreign thread, it isn’t actually metric. This is why it comes in imperial sizes: 1/8, 1/4, 1/2, 3/4, and so on.BSP parallel threads commonly seal via a 30° chamfer on the male thread to a 30° recessed cone inside the female thread (swivel only). If it is a port application, an O-ring and washer or a bonded washer are needed to achieve a proper seal for parallel threads. For tapered BSP threads, a seal is acquired via thread wedging with additional support from thread dope or Teflon tape. As previously mentioned, it is possible for a male BSPT (tapered) to thread into a female BSPP (parallel), so long as the female thread is fixed and not swivel (this is because of the recessed cone seat).BSP parallel fittings and their specifications can be identified by a completing few calculations. To find the thread size:
• Measure the O.D (outer diameter) of the BSP thread
• Take the O.D measurement (in inches) and subtract 1/4 inch (.25”).
For example, a BSP parallel male thread measures out to O.D 1”. Subtracting 1/4 gives a thread size of 3/4, also known as “dash” 12.
Once you have the thread size, you then need to determine the number of threads per inch to verify that it is a BSP fitting. You can do this by counting the number of thread crests over a 1/4” length, then multiply it by 4 to get the number of threads/inch. For example, if over a 1/4” distance you find 3.5 thread crests:
• Multiply 3.5 by 4 to get 14. This is the number of threads per inch.
• Combine the thread size with the number of threads per inch and refer to the following chart to find your desired size and its related specifications.