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Pipe Sizes

The British Standard Pipe (BSP) is a family of standard screw thread types that has been adaopted internationally for interconnecting and sealing pipe ends by mating an external (male) with an internal (female) thread and has been adopted as a standard scale used in plumbing fittings, except in the United Stated where NPT is the standard used.

  • Parallel ('straight') threads, British Standard Pipe Parallel thread (BSPP; originally also known as British Standard Pipe Fitting thread/BSPF and British Standard Pipe Mechanical thread/BSPM), which have a constant diameter; denoted by the letter G.
  • Taper threads, British Standard Pipe Taper thread (BSPT), whose diameter increases or decreases along the length of the thread; denoted by the letter R.

A list of 16 thread sizes are defined by the standards, ranging from 116 to 6. The size number was originally based on the inner diameter measured in inches of a steel tube for which the thread was intended, but in the modern metric version of the standard, it is simply a size number.

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