Thread: How do you measure NPT fittings???

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Pipe sizing is called out by the nominal I.D.

Few things more frustrating than figuring out pipe and tubing sizes...

Here's some good info on NPT and AN

National Pipe Thread Tapered (NPT) is one of the U.S. standards for tapered threads used to join pipes and fittings. The taper rate for all NPT threads is 1⁄16 (¾” per foot) measured by the change of diameter (of the pipe thread) over distance.

Nominal Pipe Size (NPS) is loosely related to the inside diameter of schedule 40 pipe, i.e. ½” is the ID of the 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.

The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. However a clearance remains between the crests and roots of the threads, resulting in a leakage around this spiral. This means that NPT fittings must be made leak free with the aid of thread seal tape or a thread sealant compound. (The use of tape or sealant will also help to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.)

The AN thread is a particular type of fitting used to connect flexible hoses and rigid metal tubing that carry fluid. It is a US military-derived specification stemming from a joint standard agreed upon by the Air Force and Navy, hence AN.

AN sizes range from -2 (dash two) to -32 in irregular steps, with each step equating to the OD (outside diameter) of the tubing in 1/16" increments. Therefore, a -8 AN size would be equal to 1/2" OD tube (8 x 1/16 = 1/2). However, this system does not specify the ID (inside diameter) of the tubing because the tube wall can vary in thickness.

Each AN size also uses its own standard thread size. SAE Thread Size for AN:-2=5/16-24, -3=3/8-24, -4=7/16-20, -5=1/2-20, -6=9/16-18, -8=3/4-16, -10=7/8-14, -12=1-1/16-12, -16=1-5/16-12
Regards All,
Glenn