BSP vs NPT
NPT threads are common in the United States and a few other countries, BSP threads (also know as the R subset according to ISO 7) are widely used in many other countries.
BSPT -British Standard Pipe Taper
BSPP -British Standard Pipe Parallel (G)
NPT -National Pipe Taper
NPS -National Pipe Straight
R - External taper (ISO7)
Rp - Internal Parallel (ISO7/1)
Rc - Internal Taper (ISO 7)
Rs - External Parallel
While the actual specified outside diameters of American National Pipe differ slightly from those of British Standard Pipe, either thread may reliably be cut onto a pipe of its respective trade size. BSPT equivalent is NPT and BSPP’s equivalent is NPS.
Never swap threads if it is a high pressure application.
NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most sizes have a different pitch. NPT/NPS threads have a 60° angle and have flattened peaks and valleys (Sellers thread form) where as BSP threads have a 55° angle and have rounded peaks and valleys (Whitworth thread form).
NPT and BSP thread pitches (threads per inch) are listed below. To determine pitch, use a thread gauge or count the number of threads that fall into a 1" span.