"AN" Thread Sizes (Army/Navy Threads)

Commonly used on Nitrous Oxide systens. The sizing chart and numbering system was deviloped during World War II where the armed forces used Nitrous Oxide systens to boost the power output of aircraft engines.

AN sizes, originally developed for use by the U.S. Armed forces ("A" for army and "N" for navy), describe the outside diameter (O.D.) of tubing in 1/16-inch increments. For example, an AN 2 fitting will fit a tube with an O.D. of 2/16", or 1/8", while an AN 8 fitting will fit a tube with an O.D. of 8/16", or 1/2". Because the actual thickness of tube walls can vary from brand to brand, the inside diameter of a tube is not used as a reference. You will also find the dash (-) symbol or the word "dash" itself used in conjunction with AN sizes. A "dash six" fitting translates to AN-6.

Each AN fitting has an established thread sizing. The following chart shows the relationship between AN size, tube O.D., and SAE thread size:

-AN
SIZE
Metal Tube
O.D. Inches
Closest SAE
Thread Size
-2 1/8" 5/16-24
-3 3/16" 3/8-24
-4 1/4" 7/16-20
-5 5/16" 1/2-20
-6 3/8" 9/16-18
-8 1/2" 3/4-16
-10 5/8" 7/8-14
-12 3/4" 1-1/16-12
-16 1" 1-5/16-12
-20 1-1/4" 1-5/8-12
-24 1-1/2" 1-7/8-12
-28 1-3/4" 2-1/4-12
-32 2" 2-1/2-12
NOTE: A sealer is NOT required for "AN" type fittings.

"NPT" Thread Sizes (National Pipe Taper)

NPT sizes are the most commonly used fitting sizes for general plumbing, piping, and tubing use; not quite as popular as AN for automotive use, but still very common. While AN fittings depend on the outside diameter of a tube for sizing, NPT fittings depend on the interior diameter (I.D.) of the fitting itself. The following chart shows the each size's thread-per-inch count, the I.D. of the fitting, and the AN fitting size with the closest-matching I.D.(inside dimension).

Pipe
Thread Size
Threads
Per Inch
App. Interior
Diameter
Closest
AN Size
1/16" 27 1/16" 2
1/8" 27 1/8" 4
1/4" 18 1/4" 6
3/8" 18 3/8" 8
1/2" 14 1/2" 10
3/4" 14 3/4" 12
1" 11-1/2 1" 16
1-1/4" 11-1/2 1-1/4" 20
1-1/2" 11-1/2 1-1/2" 24
2 11-1/2 2" 32
NOTE: A sealer is required for "NPT" type fittings.
Inch (SAE) To Metric Conversion
inch 1000th / inch mm
1/64 0.016 0.397
1/32 0.031 0.794
3/64 0.047 1.191
1/16 0.063 1.588
5/64 0.078 1.984
3/32 0.094 2.381
7/64 0.109 2.778
1/8 0.125 3.175
9/64 0.141 3.572
5/32 0.156 3.969
11/64 0.172 4.366
3/16 0.188 4.763
13/64 0.203 5.159
7/32 0.219 5.556
15/64 0.234 5.953
1/4 0.250 6.350
17/64 0.266 6.747
9/32 0.281 7.144
19/64 0.297 7.541
5/16 0.313 7.938
21/64 0.328 8.334
11/32 0.344 8.731
23/64 0.359 9.128
3/8 0.375 9.525
25/64 0.391 9.922
13/32 0.406 10.319
27/64 0.422 10.716
7/16 0.438 11.113
29/64 0.453 11.509
15/32 0.469 11.906
31/64 0.484 12.303
1/2 0.500 12.700
33/64 0.516 13.097
17/32 0.531 13.494
35/64 0.547 13.891
9/16 0.563 14.288
37/64 0.578 14.684
19/32 0.594 15.081
39/64 0.609 15.478
5/8 0.625 15.875
41/64 0.641 16.272
21/32 0.656 16.669
43/64 0.672 17.066
11/16 0.688 17.463
45/64 0.703 17.859
23/32 0.719 18.256
47/64 0.734 18.653
3/4 0.750 19.050
49/64 0.766 19.447
25/32 0.781 19.844
51/64 0.797 20.241
13/16 0.813 20.638
53/64 0.828 21.034
27/32 0.844 21.431
55/64 0.859 21.828
7/8 0.875 22.225
57/64 0.891 22.622
29/32 0.906 23.019
59/64 0.922 23.416
15/16 0.938 23.813
61/64 0.953 24.209
31/32 0.969 24.606
63/64 0.984 25.003
1 1.000 25.400