Generally the high pressure paint hose has two types of thread.
NPT threads are common in the United States, While BSP threads are widely used in many other countries.
BSPT -British Standard Pipe Taper
BSPP -British Standard Pipe Parallel
NPT -National Pipe Taper
NPS -National Pipe Straight
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 and BSPP threads are analogous to NPT and NPS threads, respectively.
WARNING: Never, never try to mate a BSP fitting with an NPT or NPS fitting if the pressure holding capability is at all critical.
NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most diametrical sizes have a different pitch. NPT/NPS threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form); BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form).
NPT and BSP thread pitches (threads per inch, TPI) are listed below. To determine pitch, use a thread gauge or count the number of threads that fall into a 1″ span. Note that, strictly speaking, when we use threads per inch, we are actually specifying the inverse of the pitch, pitch being in units of [length] / [peak to peak]. Metric threads are usually specified in actual pitch, e.g., 1.5mm, 2.0mm, etc. This is the actual length of each thread, peak to peak. Although the term “pitch” is universally used, albeit loosely, to describe threads per inch, the actual pitch of a 1/4BSP fitting is really 1/19 inch, or 0.0526 inches.
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