Gasket and AN Fitting Size Converter

Understanding AN Fitting and Dash Sizes

AN (Army-Navy) fittings are a standardized system of fittings and adapters originally developed for military aircraft in the United States during World War II. They are now widely used in automotive racing, aerospace, hydraulic systems, and high-performance plumbing applications. The sizing system uses “dash sizes” that correspond to the tube outside diameter (OD) in sixteenths of an inch.

The Dash Size System:

The dash number indicates the tube OD in sixteenths of an inch:

Tube OD (inches) = Dash size / 16 Dash size = Tube OD (inches) × 16

For example, a -6 (dash six) fitting fits a tube with 6/16 = 3/8 inch OD.

AN Fitting Size Reference Table:

Hose ID vs Tube OD:

The hose inside diameter (ID) is always slightly smaller than the tube OD for the same dash size. This is because hoses fit OVER fittings, while tubes fit INTO fittings. The step is not one fixed figure:

Hose ID (inches) = Tube OD - 1/32 below -8, and - 1/16 from -8 up

The one exception is -32, which drops a full 1/8 inch. Take the ID from the table above rather than doing the subtraction yourself; the calculator does exactly that and only falls back to the rule for an odd tube size that is not a standard dash.

Common Applications by Dash Size:

Practical Examples:

• A -6 AN fuel line uses tubing with 3/8 inch (9.53 mm) OD and hose with 0.344 inch (8.73 mm) ID.
• Converting a standard 1/2 inch brake line to AN fittings requires -8 AN components.
• A turbo oil feed line typically uses -4 AN (1/4 inch tube OD, 6.35 mm) fittings and lines.

Where the seal actually is (and where the gasket lives):

An AN fitting has no gasket. The seal is metal on metal: a 37-degree flare on the tube pressed against a 37-degree cone on the fitting, and the nut behind it supplies the clamping force. Nothing soft is involved, which is why AN plumbing tolerates fuel, oil, brake fluid and heat that would destroy a rubber seal, and why over-tightening does not help. If a flare leaks, the flare is wrong.

The gasket appears at the other end of the fitting, where it screws into a port. A straight-thread O-ring boss port, marked ORB, carries an O-ring that squashes against a flat face on the boss. Those O-rings come from the AS568 900-series, whose numbers track the dash size, so the port dash number is what you order by. Do not confuse that with a tapered NPT port, which seals on the thread itself and takes sealant rather than an O-ring, and never wrap PTFE tape around an ORB or a flare.

Tips:

• AN fittings use 37-degree flared seats, and JIC fittings use the same 37-degree flare. They are often interchangeable but always verify the standard, because the thread pitches diverge on some sizes.
• Never mix AN (37-degree) with SAE 45-degree flare fittings. They look similar and do not seal properly together.
• Anodised aluminium fittings are usually two-tone red and blue on every size. That colouring is decoration, not a size code. The size is stamped on the hex or printed on the packaging, so read it rather than counting on the colour.
• Aluminium hexes are soft. Use an AN wrench, or at worst a good flare wrench, and expect to round the corners with an adjustable.