AEM -6AN Discharge Fitting

Thread Types

AN Standards
High-end racing and performance fuel delivery systems most frequently implement fuel lines and fittings manufactured to AN Standards. These standards are headed under “AN” which is the abbreviation for “Army-Navy”, but is also known as “Air Force – Navy Aeronautical Standard”. After the war many used surplus aerospace parts were used for racing, and thus introduced the AN standard into the racing and performance world.

AN Standards as Applied to Fluid Carrying Tubes and Flexible Hoses
In the case of tubes and flexible hoses used to carry fluid, “AN Size” designates the outer diameter of the hose or tubing in 1/16” increments. For example an AN size 2 designates a tube OD of 1/8” (2 x 1/16”), the tube OD of AN size 3 is 3/16” (3 x 1/16”). Note: The AN size number is correctly displayed with a dash before the number.

FYI: Popular fuel line sizes:
-6AN (=3/8″ OD)
-8AN (=1/2″ OD)
-10AN (=5/8″ OD)
-12AN (=3/4″ OD)

Use of -3AN and -4AN line:
Typically, these 1/4″ and 5/16″ line sizes are in use for low flow or OEM systems, but are commonly used in high performance applications for pressure gauges as well as plumbing between the Regulator and Intake Manifold. These two lines do not involve significant amounts of flow, and therefore do not require larger sizes. Typically, the port sizes used with these smaller sizes use a 1/8″- NPT. 1/8″-NPT fittings are very common and can easily be used to adapt to -3AN or -4AN line.

AN Standards as Applied to Fluid Carrying Fittings
Each AN size also references a standard male and female thread size that is used on fittings – such as hose end fittings, adapters, and port fittings. This is known as “AN Thread Size”. AN fittings have Straight Thread, and use O-ring seals. The O-ring size is typically defined by the “AS” standard. Dash Numbers are typically assigned based on the size. Boss-Seal style O-rings are typically in the “900” Series. For example -906 is for -6AN ports, and -908 is for -8AN ports.

AN Size and AN Thread Size Reference ChartThe chart below references the tube (or flexible hose) OD and thread size for each AN size.

Common Types of Fittings

Adapter Fittings
Adapter Fittings are commonly used to connect fuel line tubes and hoses to fuel pumps, fuel regulators and fuel filters. While Adapter Fittings are available in SAE size configurations, most high-end racing and performance fuel systems use those manufactured to AN standards. With AN versions, the fitting ends typically use a 37° cone on each end. AN standards dictate use of Straight Thread for Adapters, requiring use of O-rings. Special port machining is done to “capture” and properly compress the O-ring, as too much stress can damage the O-ring, and too little can cause leakage. These port standards are designed to precisely “load” the O-ring automatically during installation.

Pipe Fittings
Pipe Fittings can be used to connect fuel lines to the port on a fuel pump or fuel pressure regulator, as well as connect fuel pressure gauges to gauge ports. One end (the port end) of the fitting can be described as a male open-ended cylinder with Tapered Thread (also called Pipe Thread) on the outer diameter. This fits into a female port of corresponding size and thread pitch. The other end typically uses a 37° cone and Straight Thread. The Tapered Thread on the port end of the fitting must use sealing compound or Teflon tape to help avoid leakage.

Port Fitting
Port Fittings function just like Pipe Fittings but with the following distinctions. Port Fittings conform to AN standards. The cone end uses Straight Thread and a 37° cone. The male port end of the fitting uses Straight Thread, and is received by a female port of corresponding size and thread pitch. Port Fittings require use of O-rings. Like AN Adapters, special port machining is done to “capture” and properly compress the O-ring which allows for greater sealing ability with very high reliability of installation as compared to Tapered Threads using sealing compounds or Teflon tape. This reliability makes Port Fittings the preferred choice over Pipe Fittings for high-end racing and performance fuel system applications.