AN Fitting Sizes Explained: The Complete Guide to Choosing the Right AN Fittings

If you're plumbing a fuel system, oil cooler, or catch can setup, sooner or later you'll run into the same question every builder faces: what do AN fitting sizes actually mean, and which ones does your car need? Get it right and you end up with a leak-free, professional-grade fluid system that looks as good as it flows. Get it wrong and you're chasing drips, cross-threaded flares, and starved fuel rails. This guide breaks down AN fitting sizes in plain English — the dash numbers, the thread specs, the hose end types — so you can order once and wrench once.

What the Dash Numbers in AN Fitting Sizes Actually Mean

AN sizing comes from a military spec (Army-Navy, hence the name) originally written for aircraft plumbing. The dash number refers to the tube outside diameter in sixteenths of an inch. A -6AN fitting corresponds to 6/16" — that's 3/8" tube OD. A -8AN is 8/16", or 1/2". A -10AN is 5/8", and so on.

The quick-reference numbers most street and track builds care about:

• -4AN (1/4" tube OD): brake lines, gauge feeds, turbo oil feed lines
• -6AN (3/8" tube OD): fuel feed and return on most street builds, catch can lines
• -8AN (1/2" tube OD): higher-horsepower fuel systems, oil cooler lines on smaller engines
• -10AN (5/8" tube OD): oil cooler and remote oil filter plumbing, high-flow fuel setups
• -12AN and up (3/4"+ tube OD): dry sump systems, radiator hoses, big-power fuel cells

One critical detail: the dash number describes the tube the fitting was designed for, not the hose's inside diameter or the wrench size. Always match dash number to dash number — a -6AN hose end goes with -6AN hose and a -6AN adapter, period.

Flare, ORB, and NPT: Know Your Seal Before You Order

The 37-degree flare is the heart of the AN standard. The male fitting's coned flare seats against the female fitting's matching cone, and the metal-to-metal contact makes the seal — no tape, no sealant. That's different from the 45-degree flares used on some SAE automotive fittings, and the two do not mix. Mating a 37-degree flare to a 45-degree seat will seal just long enough to let you down.

You'll also run into two other thread types on the same project. ORB (O-ring boss) fittings seal with an O-ring on a straight thread — common on fuel rails, pumps, and regulators — and a swivel adapter like the RaceFlux 90-degree ORB to AN male flare adapter converts that port to a standard AN flare. NPT (tapered pipe thread) seals on the threads themselves and needs sealant; a 90-degree male NPT to AN flare adapter is the usual bridge from a block or pump port into AN plumbing. Identify the port first, then buy the adapter — most "leaky AN fitting" complaints are actually a mismatched thread type.

What Size AN Fitting for Fuel Line? A Practical Starting Point

The most common question we get is what size AN fitting for fuel line duty. For a naturally aspirated street build up to roughly 400 horsepower, -6AN feed with a -6AN return is the proven baseline. Push past that — boost, E85, big injectors — and stepping the feed up to -8AN keeps the pump from working against restriction. E85 in particular demands roughly 30 percent more fuel volume than gasoline, so flex-fuel builds should size up early rather than re-plumb later.

Oil systems run bigger. Oil is thicker than fuel, and restriction costs you pressure where it matters, so oil cooler and remote filter lines are typically -10AN. If you're planning that project, our engine oil cooler kit guide covers line routing and cooler sizing in depth, and catch can plumbing pairs naturally with the fittings on this page — see the oil catch can guide for the full setup.

Choosing Hose Ends: Angles, Swivels, and PTFE

Hose ends do the real work in a clean install. The RaceFlux lineup in our AN fittings collection covers every angle from straight through 180 degrees, and two features are worth prioritizing:

• Double-swivel design: a hose end like the RaceFlux 90-degree double swivel seal hose end rotates after assembly, so you can clock the fitting perfectly without twisting the hose.
• PTFE-lined hose ends: for E85, race gas, brake fluid, or power steering, PTFE liner is the safe choice — it won't degrade or weep fuel smell the way rubber-lined hose can. The PTFE straight hose end in stainless steel is the workhorse here.
• Angle selection: use 45s and 90s to route around obstacles instead of forcing hose into tight bends that kink and restrict flow.
• Unions and couplers: joining two hose sections or extending a run is a one-piece job with an AN male flare union.

Routing through a firewall or fuel cell wall? That's what bulkhead fittings are for — they clamp through the panel and give you a sealed AN port on both sides. If you're chasing a show-quality engine bay while you're in there, our brake line tuck guide shows what clean AN plumbing looks like in practice.

Pro Tip: Assemble It Right the First Time

Aluminum AN fittings are anodized, and a steel wrench will chew that finish — use aluminum AN wrenches or tape the jaws. Before final assembly, put a drop of engine oil on the flare and threads (never thread sealant on a flare — it prevents proper seating). Tighten until the fitting seats firmly, then a final snug; AN seals on the flare cone, not on gorilla torque. After the first heat cycle, re-check every connection. A properly assembled 37-degree flare seals dry and can be taken apart and reused for years.

Get Your Fuel and Oil Plumbing Done Right

Whether you're plumbing a full fuel system or just adapting one stubborn port, the right fitting is in stock. Browse the complete AN fittings collection for hose ends, adapters, unions, and bulkheads, or shop the full RaceFlux lineup for clamps, quick-connects, and hardware to finish the job. Order what your build needs today and plumb it once, properly — your future self under the car will thank you.