What are AN fittings?

"AN" fittings are a standardized system for tubing and fittings originally created for the U.S. military (Army-Navy). Designations such as AN-8 are Army-Navy sizes. In theory, fittings and parts with the same AN size will always fit together.

For a given AN-x size, take x/16" to get the simple imperial size. For example, an AN-8 fitting is 8/16" (= 1/2") big. An AN-4 fitting is 4/16" (= 1/4") big.

Sizes refer to the outside size only. The inside size (bore) of AN fittings is not standardized, so some parts will have a smaller inside diameter than others.

AN fittings are sometimes used in the automotive world as upgrades to existing factory parts. In the case of DSMs, the metric-to-AN adapters can sometimes be hard to come by. Some people substitute combinations of metric-to-NPT (national pipe thread) and NPT-to-AN fittings - both types are airtight when torqued correctly.