AN & NPT thread sizes - AN thread sizes

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.

AN fittings are a flare fitting, using 37° flared tubing to form a metal-metal seal. They are similar to other 37° flared fittings, such as JIC, which is their industrial variant. The two are interchangeable in theory, though this is typically not recommended due to the exacting specifications and demands of the aerospace industry. The differences between them relate to thread class and shape (how tight a fit the threads are), and the metals used.

Note that 37° AN and 45° SAE (commonly found on household plumbing adapters) fittings and tooling are not interchangeable due to the different flaring angles. Mixing them can cause leakage at the flare.

AN & NPT thread sizes - NPT thread sizes

National Pipe Thread Tapered Thread (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings. The taper rate for all NPT threads is 1/16 (¾ inch per foot) measured by the change of diameter (of the pipe thread) over distance. The taper divided by a center line yields an angle 1° 47' 24" or 1.7899° as measured from the center axis. Commonly-used sizes are 1/8, 1/4, 3/8, 1/2, 3/4, 1, 1-1/4 1-1/2, and 2 inch, appearing on pipes and fittings by most U.S. suppliers. Smaller sizes than 1/8 are occasionally used for compressed air. Sizes larger than 2 inch are uncommon because other methods of joining are more practical in most applications. It is defined by ANSI/ASME standard B1.20.1.

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.

Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid. These threads are now used in materials other than steel and brass, including PTFE, PVC, nylon, bronze and cast iron.

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.)

AN fittings wrench sizes

  3 AN Wrench Head (1/2" Hex Size)
  4 AN Wrench Head (9/16" Hex Size)
  6 AN Wrench Head (11/16" Hex Size)
  8 AN Wrench Head (7/8" Hex Size)
10 AN Wrench Head (1" Hex Size)
12 AN Wrench Head (1-1/4" Hex Size)
16 AN Wrench Head (1-1/2" Hex Size)