British Standard Pipe fittings are among the most popular of all foreign threads in the world today. These threads come in two versions: parallel (BSPP), and tapered (BSPT). The thread flank angle for both tapered and parallel British threads is 55°. 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, etc.
BSP parallel (BSPP) threads commonly seal via a 30° chamfer on the male thread to a 30° cone seat inside the female thread (swivel only). If it is a port application, an O-ring and washer, Elastomeric seal or a bonded washer are needed to achieve a proper seal for parallel threads. For tapered BSP threads (BSPT), a seal is accomplished via thread wedging with additional support from thread dope or 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 the recessed cone seat could interfere with clearance). To identify a BSPP fitting complete the following steps:
Confirm the thread is parallel or tapered. Determine the threads per inch (TPI) count using a thread gauge or calipers. Determine the thread O.D (male thread) or I.D (female thread) using calipers. Find a match on the adjacent chart by combining your O.D or I.D measurement with the TPI you measured. If a seal or angled sealing surface exists, use it to identify which specific type of British fitting it is on the chart to the right. To simplify this process, try Adaptall’s TGK (thread gauge kit).Use the diagram to your right to confirm what type of British fitting you he. Use caution as “multi-purpose fittings” exist that accommodate 2 or more sealing styles in one fitting (such as Adaptall’s 9005ES series that can be used for BSPP port or swivel connection).
BSPP Thread Specification Chart Dash Size Nom. Thread Size Threads per inch (TPI) Male Thread O.D (inches) Male Thread O.D (MM) Female Thread I.D (MM) Female Thread I.D. (inches) 02 1/8 28 9.6 0.38 8.6 0.34 04 1/4 19 13 0.51 11.9 0.47 06 3/8 19 16.5 0.65 15.2 0.6 08 1/2 14 20.8 0.82 19.1 0.75 10 5/8 14 22.8 0.9 20.8 0.82 12 3/4 14 26.3 1.04 24.6 0.97 16 1 11 33.1 1.3 30.7 1.21 20 1-1/4 11 41.8 1.64 39.4 1.55 24 1-1/2 11 47.7 1.88 45.5 1.79 32 2 11 59.5 2.34 57.4 2.26 40 2-1/2 11 75.1 2.95 72.6 2.86 48 3 11 87.9 3.46 85.4 3.36 BSPP Threads and Sealing Methods