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 that dates back to World War II and stems from a joint standard agreed upon by the Army 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.
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 AN threads are different for bolts and fittings. In bolts the number refers to the diameter of the bolt whereas in a fitting it refers to the OD of the tube and thereby have different threads. For example AN6 bolt has a 3/8-24 thread  whereas an AN6 fitting has a 9/16-18 thread.
Note that 37° AN and 45° SAE fittings and tooling are not interchangeable due to the different flaring angles. Mixing them can cause leakage at the flare.
Originally controlled by the specification MS33656, but is now controlled by SAE-AS4395.
|SAE thread size||5/16-24||3/8-24||7/16-20||1/2-20||9/16-18||3/4-16||7/8-14||1-1/16-12||1-5/16-12||1-5/8-12||1-7/8-12||2-1/4-12||2-1/2-12|
|Pipe thread size (NPT)||1/8-27||1/4-18||3/8-18||1/2-14||3/4-14|
- Coastfab http://www.coastfab.com/images/pdf/2010/bolts_an3_an10.pdf. Retrieved 10 July 2014. Missing or empty
- Holley.com http://www.holley.com/data/TechService/Technical/Thread%20Sizes.pdf. Retrieved 10 July 2014. Missing or empty
- Smith, Carroll (1984). Engineer to Win. Motorbooks International. p. 184. ISBN 978-0-87938-186-8.