S-LOK STAINLESS STEEL FITTINGS

National Pipe Thread NPT thread, American NPT tapered thread ASME/ANSI B1.20.1´. This self-sealing tapered thread is standard in the United States for connecting air lines.
NPT (National Pipe Thread) fittings are the most used type of connection in the USA and Canada. NPT fittings have a tapered thread, which fits into the female NPT adapter. They seal by virtue of the "out of round" principle, which means that the male thread extends the female connection until there is sufficient force in the connection to hold pressure. One of the disadvantages of this operation is that when high quality steel is connected to non-rusting steel, excessive stress or poor lubrication can cause damage to the thread. There are waterproofed sealing threads, but they only need 2 rotations.

BSPT fittings

Tapered Threads, British Standard Pipe Taper (BSPT) Thread: This tapered thread is self sealing, but can also be used with sealant.
BSPT is similar to NPT, but there is one important difference. The thread angle is 55 degrees instead of the usual 60 degrees for NPT. So, an NPT fitting will fit into a BSPT connection or vice versa, but they won't be sealed. Threaded fitting sealants are required to seal the male and female connectors. BSPT fittings are a common connection method in China and Japan, but very rarely used in North America unless the equipment to which they are connected has been imported.

BSPP fittings

Cylindrical Threads (Straight), British Standard Pipe Parallel Thread (BSPP): This cylindrical thread should be sealed with sealant or a washer.
BSPP is common in Great Britain, Europe, Asia, Australia, New Zealand and South Africa. It is a cylindrical thread, which requires an O-ring or sealant to make it waterproof. This gasket or sealant is inserted between the male and female fitting and compressed into place.