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British Pipe Standard fittings are amongst the most popular of all foreign threads in the world. These threads come in 2 versions: parallel (BSPP), and tapered (BSPT). The thread flank angle for both tapered and parallel British threads is 55°. NOTE: It is a common mistake to identify a BSPT (tapered) as an NPT. 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 metric. This is why it comes in imperial sizes: 1/8, 1/4, 1/2, 3/4, and so on. 
BSPP-tech 

  • •Thread conforms to ISO 228-1
  • •Port conforms to ISO 1179
  • •Pitch & diameter measured in inches e.g. G1/4-19
  • •Parallel threads require o-ring, crush washer, gasket or metal to metal seal between connectinos for pressure tight connection
  • •Thread angle is 55°
  • Not interchangeable with SAE or NPT(F)
BSPP Thread Specification
(BSPP Indicator: G)
SizeNominal Size (in)Thread Size (mm)
2 1/8 9.728 - 28
4 1/4 13.157 - 19
6 3/8 16.662 - 19
8 1/2 20.955 - 14
10 5/8 22.911 - 14
12 3/4 26.441 - 14
14 7/8 30.201 - 14
16 1 33.249 - 11
18 1-1/8 37.897 - 11
20 1-1/4 41.910 - 11
22 1-3/8 44.323 - 11
24 1-1/2 47.803 - 11
28 1-3/4 53.746 - 11
32 2 59.614 - 11
Thread Size = Male Thread OD - Threads per Inch

2) British Standard Pipe Tapered


For tapered BSP threads, a seal is acquired via thread wedging with additional support from thread dope or Teflon® tape. 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 on the BSPP end). 

You must find the I.D. to determine the threads per inch count for BSPT fittings. Simply obtain a caliper reading from inside the bore of the fitting. You can cross-reference the bore size on the following chart with thread pitch and threads per inch to find your desired size and its related specs. 
BSPT-tech

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