What Is an AN Compression Fitting and How Does It Work?

An AN fitting seals fluid with two 37° flared metal faces pressed together — the threads only supply clamping force, and AN sizes track tube outside diameter in sixteenths of an inch.

A fuel line that weeps at a joint is usually a sealing problem, not a torque problem, and that single idea explains most of how these fittings behave. An AN compression fitting is a flare-style connector built to the Army-Navy sizing system, and once you understand that the metal cone does the sealing while the nut does the holding, the whole design stops looking fussy and starts looking sensible. Here’s the sizing logic, the thread numbers, and the mistakes that cause leaks.

What Does the “AN” in AN Fitting Actually Mean?

The letters stand for Army-Navy, a military-derived standard that migrated into automotive, racing, aerospace, and hydraulic work, where it remains the default for high-pressure fluid lines. The sizing is the part people trip on: AN numbers describe the tube’s outside diameter in sixteenths of an inch, not the thread and not the flow.

That one rule decodes the entire catalog. A 6AN line is 6/16 in. — 3/8 in. tube. A 10AN is 5/8 in. Nothing about the number tells you the wrench size or the thread pitch, which is why a chart matters before you order.

An AN Fitting Size and Thread Chart

Match the AN dash number to the thread size before assembly, because the two scales don’t move together in an obvious way. The table below pairs the common sizes with their tube dimensions and thread specs.

Pegasus Auto Racing maintains a widely used reference chart pairing AN hose and fitting sizes with their matching thread sizes, and it’s the kind of document worth bookmarking before a build. SAE’s J516 standard, which covers hydraulic hose fittings, is the formal backbone behind a lot of this hardware. If you’d rather skip the sourcing and just compare what’s actually available, this roundup of tested AN compression fitting options covers the common sizes and connection types.

How Does the Seal Actually Form?

The seal happens at the flared interface, where a male 37° cone seats against a matching female flare face and the nut clamps the two together. Fluid never reaches the threads in a correctly assembled joint — they exist to generate clamping force along the axis, which is exactly why wrap-style thread sealant is the wrong tool here.

A few practical points follow from that geometry:

• Keep PTFE tape and heavy thread sealants off the threads of these connectors. They can’t help, because the threads aren’t the sealing surface, and stray tape fragments can migrate into a fuel or oil system.
• Protect the conical faces. A scratch, dent, or speck of grit on the 37° surface is a leak path, and no amount of tightening will close it.
• Tighten to clamp the flares together — snug and even, not gorilla-tight. The faces do the work.
• Inspect the assembly and pressure-test it after building, before it goes into service.

A basic assembly sequence runs: secure the female connector, insert and advance the hose, then tighten the male connector to clamp the flares. Every product variant differs by connection type — hose end, tube end, or adapter — so confirm which one your application calls for before assembly begins.

AN vs NPT: Why They Can’t Be Mixed

NPT and AN are different thread forms with different sealing methods, and treating them as interchangeable is a reliable way to create a leak. NPT is a tapered pipe thread that seals on the threads themselves with a sealant. AN seals on a 37° flare face and uses its threads only for clamping force.

The sizing trap sits on top of that: choosing an AN size by the outside diameter you measured can be wrong when the real constraint is hose inside diameter or the flow the system needs. Those are different questions with different answers, which is why the sourcing chart gets checked carefully rather than eyeballed.