Hydraulic Connecting Threads speaks to pipe connecting threads and pipe connectors are presented in varying standards throughout the world and they are:
National Pipe Thread Fuel (NPTF) Straight Thread O-Ring (SAE) British Standard Pipe Tapered (BSPT) British Standard Pipe Parallel (BSPP) Metric Straight Thread O-Ring
These standards are used for tapered threads used on threaded pipes and fittings with imperial sizes used in the USA and metric in the rest of the industrialized world. Sizes smaller than 1/8 inch or 125 mm are used for compressed air while sizes larger than 2 inches or 2000 mm are not common and alternative methods are used for these larger sizes.
Nominal Pipe Size is loosely related to the inside diameter of Schedule 40 pipe. Because of the pipe wall thickness, the actual diameter of the threads is larger than the NPS, considerably so for small NPS. Other schedules of pipe have different wall thickness but the OD (outer diameter) and thread profile remain the same, so the inside diameter of the pipe is therefore different from the nominal diameter.
National Pipe Thread Fuel (NPTF)
Thread conforms to ANSI B1.20.3 Physically interchangeable with NPT but has modified threads for better pressure tight sealing. Tapered thread profile seals by metal to metal interference fit, usually requires sealing compound for pressure tight connections. Pitch and diameter are measured in inches. Taper angle is 0.75” per foot or 1 Degree 47 ft. Thread angle is 60 degree.
Straight Thread O-Ring (SAE)
Thread conforms to ISO 263 and ANSI B1.1 Unified Port conforms to ISO 11926 and SAE J1926 Commonly called straight thread O-ring fittings. Pitch and diameter are measured in inches, 1/16 – 12 UN-2B Threads are parallel and require O-ring for pressure tight connection Thread angle is 60 degrees
British Standard Pipe Tapered (BSPT)
Thread conforms to ISO 7 Pitch and diameter are measured in inches e.g. G3/8-19 Tapered thread profile seals by metal to metal interference fit, usually requires sealing compound for pressure tight connection Tapered angle is 1 degree 47 feet, the same as NPT(F) Thread angle is 55 degrees Not interchangeable with NPT(F)
British Standard Pipe Parallel (BSPP)
Thread conforms to ISO 228-1 Port conforms to ISO 1179 Pitch and diameter measured in inches e.g. G1/4-19 Parallel threads require O-ring, crush washer, gasket or metal to metal seal between connections for pressure tight connection Thread angle is 55 degree Not interchangeable with SAE or NPT(F)
Metric Straight Thread O-Ring
Thread conforms to ISO 261 Port conforms to ISO 6149 and SAE J2244 Pitch and diameter measure in millimeters e.g. M22 X 1.5 Parallel threads require O-ring for pressure tight connection Thread angle is 60 degrees Easily identifies by raised ridge on female counterbore Not interchangeable with SAE or BSPP