AN fittings and fuel lines

AN stands for Army Navy. The fittings came from military aircraft and became the standard for race car plumbing. The size is written as a dash number, like -6, which you say as dash six.

The dash number is the outside diameter of the matching hard tube in sixteenths of an inch. A -6 is six sixteenths, which is 3/8 inch. A -8 is 1/2 inch. A -10 is 5/8 inch.

For hose, the dash number names the hard tube the hose replaces. The inside of the hose is close to that size but not exact. A -8 hose has an inside diameter of roughly 0.440 inch.

An AN joint seals where a 37 degree male cone meets a 37 degree female seat. Tightening the nut presses the two metal surfaces together. The seal is metal to metal.

The threads only hold the parts together. They do not seal anything. That is why AN flare threads never get thread sealant or tape. Sealant does nothing for the flare seal. Shreds of tape can break loose and plug a filter, a pump or an injector.

Never mix AN fittings with 45 degree flare fittings. The 45 degree flare is common on brake lines and household plumbing. The angles do not match, so the joint leaks and both parts can be damaged.

Tighten an AN fitting to the maker's spec and no further. Overtightening can crack or gall the flare. A damaged flare will not seal.

Wrap tape around the hose where you will cut it so a braided hose does not fray. Cut it square with a hose cutter or a fine cut-off wheel. Trim any loose strands.

Push the hose into the socket with a twisting motion until it bottoms. Mark the hose at the edge of the socket so you can see if it moves. Put a light coat of oil on the nipple threads and the inside of the hose.

Hold the socket in soft vise jaws. Thread the nipple in with an AN wrench. AN wrenches are aluminum so they do not mar the anodized finish. Tighten until the gap between the socket and the nipple hex matches the maker's instructions. Check your mark to make sure the hose did not push back out.

Blow the finished hose out with clean compressed air. Cutting leaves debris inside, and debris can ruin a pump or plug an injector. Tug hard on each end. If a fitting moves at all, take it apart and start over.

Many pumps, regulators and engine blocks do not have AN ports. They have NPT pipe threads or ORB ports. You need the right adapter to go from either one to AN.

NPT stands for National Pipe Thread, and its threads are tapered. Tapered threads wedge tight but do not seal on their own. The NPT side of an adapter needs a thread sealant rated for fuel. Keep sealant off the first thread so none gets inside the line.

ORB stands for O-ring boss. It uses a straight thread and an O-ring that seals against the port. ORB needs no thread sealant. Use a new O-ring made for the fuel you run. Give it a light coat of clean oil before you install it.

NPT sizes are based on the inside of a pipe, so an NPT port is larger than its fraction suggests. A 3/8 inch NPT port can feed a -8 line. Pick an adapter that does not choke the flow.

Check every fitting for wet spots, stains or drips with the fuel system under pressure. A damp hose cover next to a fitting means a leak. Fix every leak before the car runs.

Look over the full length of each hose. Replace a hose that is cut, cracked, blistered, soft, hard or burnt. Replace a hose with braid or reinforcement showing through the cover. Replace a hose that has slipped in its socket.

Route hoses away from headers, turbos and exhaust, because heat shortens hose life. Keep hoses from rubbing on frame rails, brackets and sharp edges. Use cushioned clamps. Use grommets where a hose passes through sheet metal. Do not twist a hose as you tighten it. Keep every bend wider than the hose's minimum bend radius.