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AN Fittings and NPT Threads

What is an "AN" Fitting or an NPT Fitting? Hopefully this will help explain it to you.

AN Stands for 'ARMY / NAVY' and was designed as a standard by the US Military to determine common sizes of hoses and fittings. The fittings use a 37* angle at the ends to create the seal. There is also a 45* angle fitting, but in the automotive world, we use the 37* angle as the standard. Plumbing with copper pipe. The two are NOT interchangeable. The 37* fittings are sometimes referred to as "JIC" Fittings. If you never heard that term before, don't worry about it. You can get by just knowing "AN".

AN fittings are expressed in "Dash" sizes. When written, it is written with a - and not spelled out. ( -4, -6, -8 ect..) These dash sizes are expressed in a 1/16th of an inch. So if you had a "-4" line, you would have a 4/16th. As you remember from basic math classes, you can still reduce this fraction. Reduce it as much as possible and you will get 1/4.

Use the table bellow to understand each of the dash sizes as they correspond to a size and thread.

 

Dash Size
Hose Size
Thread
-3
3/16
3/8 - 24
-4
1/4
7/16 - 20
-5
5/16
1/2 - 20
-6
3/8
5/8 - 18
-8
1/2
3/4 - 16
-10
5/8
7/8 - 14
-12
3/4
1 1/16
-16
1
1 5/16
-20
1 1/4
1 5/8

In a fuel system using AN Fittings, it is very common to use an "NPT" (Nation Pipe Thread) in conjunction with the AN fittings. These NPT threads DO require the use of a tape to create the seal, where the AN fitting do NOT. When using an NPT fitting, it is common for you to only get a few turns out of the fitting. DO NOT try to be He-Man and continue tightening. It is not going to make a better seal the more you tighten the fitting. The thread is tapered and is not meant to go "all the way in" as you may think. The chart bellow should help you in explaining NPT threads.

Pipe Thread
Threads per Inch (TPI)
Inside Diameter (IDA)
1/8
27
1/8
1/4
18
1/4
3/8
18
3/8
1/2
14
1/2
3/4
14
3/4
1
11 - 1/2
1