BSPP

• British Standard Pipe Parallel thread

(BSPP; originally 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.)

British Standard Pipe Parallel threads are the most popular type of thread used in the UK, Europe, Asia, Australia, New Zealand and South Africa. These are parallel thread fittings.

• Both internal and external threads are parallel threads.
• BSPP threads are not suitable joint where pressure-tightness is made on the thread. If assemblies with such threads must be pressure-tight, this should be made by compressing two tightening surfaces outside the threads, and by applying an appropriate seal.
• Threads are normally of the truncated form, with crests truncated to the limits of tolerance. There is an exception to this for internal threads, where they are likely to be assembled with external threads in accordance with ISO 7-1 (Pipe threads where pressure-tight joints are made on the threads — Part 1: Dimensions, tolerances and designation)
• For external threads, two classes of tolerances on the pitch diameter are specified; these are Class A and Class B. The choice between class A and class B depends on the conditions of application.
• The symbol for pipe thread type is G for external and internal threads.
• Designation: Internal thread - Pipe thread ISO 228- G 1/2 , External Thread - Pipe thread 1S0 228- G 1/2 A (For tolerance Class A).

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References

1. http://www.ralstoninst.com/news/story/the-difference-between-npt-bspp-and-bspt-seals
2. ISO 7-1:2000 Pipe Threads Where Pressure-Tight Joints are Made on the Threads - Dimensions, Tolerances and Designation
3. http://www.amesweb.info/Screws/BSPT-Thread-Sizes-Dimensions.aspx