AN fitting sizes

The dash number is the tube outside diameter in sixteenths of an inch. A -6 fitting is for 6/16” tube, which is 3/8”. Once that lands the table stops needing to be memorised, and the thread sizes stop looking arbitrary.

AN and JIC fittings share these dimensions — AN is the aerospace designation, JIC the industrial one, both 37° flare. They interchange. The 45° flare fittings that look similar do not, and forcing one into the other is how a fuel line weeps.

A note on the last two columns

Tube diameter and thread are from the standard. Hose bore is not: braided hose is not dimensioned by the specification the fitting is, so it varies a little between makers, and the figures below are typical rather than definitive. The “usually” column is guidance from building these cars, not a rule — the right size depends on length, bends, pressure and how much power the system is feeding.

Sizing, roughly

For a fuel system, the feed is the line that matters and the return can be one size down. AN-6 feeds most naturally aspirated and moderate boost builds. Going up to AN-8 is worth it at high power, and on E85, which needs about 40% more volume for the same power — see sizing fuel injectors.

Brake and clutch lines are AN-3 and stay AN-3. Brake fluid barely moves; what matters there is that the line does not expand, which is a hose construction question rather than a bore one.

Turbo oil feed is small — restrictor-sized, often AN-3 or AN-4 — and the drain is much larger than people expect, AN-10 being common, because oil leaving a turbo is aerated and falling under gravity alone. A drain that is too small backs oil up into the centre section and pushes it past the seals, which looks exactly like a failed turbo and is not.