AN thread

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A 37deg flare type end fitting for flexible hose ANfitting.jpg
A 37° flare type end fitting for flexible hose

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. The Army-Navy involvement is also the origin of the red/blue color combination that was traditionally used in the anodized finishing process. [1]


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 116-inch increments. Therefore, a -8 AN size would be equal to 12-inch OD tube. 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 38-24 thread [2] whereas an AN6 fitting has a 916-18 thread. [3]

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 parts were made compliant to the specification MIL-F-5509, but they are now controlled under SAE AS (Aerospace Standards) specifications AS4841 through AS4843 and AS4875.

Size comparison [4]
AN size-2-3-4-5-6-8-10-12-16-20-24-28-32
Tube OD (inches)18316145163812583411 141 121 342
Nominal hose ID (inches)18316145163812583411 141 121 342
Actual hose ID (inches, approximate)11618316145167161258781 181 381 581 34
SAE thread size 516-2438-24716-2012-20916-1834-1678-141 116-121 516-121 58-121 78-122 14-122 12-12
Pipe thread size (NPT)18-2714-1838-1812-1434-14

See also

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  1. "AN Fitting Types | ColorFittings" . Retrieved 2020-08-05.
  2. "AN3 – AN10 Series Specifications: Bolt, Machine, Aircraft, Hex Head" (PDF). Coastfab. Archived from the original (PDF) on 14 July 2014. Retrieved 10 July 2014.
  3. "AN Thread". Retrieved 21 March 2015.
  4. Smith, Carroll (1984). Engineer to Win. Motorbooks International. p. 184. ISBN   978-0-87938-186-8.