Understanding Performance Fittings – Straight and AN Threads vs. Tapered Thread

If you are confused by the nomenclature of hoses and fittings associated with racecar plumbing, you’re not alone. Fittings are often measured in terms that begin with a dash, such as a -6 or a -8. To find out more about how these systems work, we turned to Fuelab, a manufacturer of fuel pumps, filters, pressure regulators and accessories, to help clear up the issue.

This installment of “Tech Matters” will describe the different types of fittings commonly used for high-end racing and street performance applications. The fittings we will discuss are adapter fittings — also called standard, or union — port fittings, and pipe fittings, as well as thread types and size standards associated with each.

Before we get into the types of fittings, let’s first define the thread types used on the fittings, and discuss applicable AN standards.

Thread Types

Straight Thread — With straight thread, the threads run parallel cylindrically to each other. Port fittings and adapter fittings using straight thread require use of an O-ring boss seal technique to seal against leakage.

Tapered Thread

Tapered thread, also known as pipe thread, typically uses a 2-degree angle on the threads, instead of running parallel. This allows the threads to interfere and seal off. Threads must be clean and precise to make up for gaps between threads that allow for leakage. Pipe fittings, which use tapered thread, don’t use O-rings, but rather must use compounds or teflon tape to help seal the threads. National pipe thread taper (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings.

AN Standards

High-end racing and performance fuel delivery systems most frequently implement fuel lines and fittings manufactured to AN standards. During World War II, the U.S. military set specification standards for fasteners and fluid fittings used on military equipment. These standards are headed under “AN” which is the abbreviation for “Army-Navy,” but also is known as “Air Force – Navy Aeronautical Standard.” After the war, many surplus aerospace parts were used for racing, and thus introduced the AN standard into the racing and performance world.

AN Standards For Fluid-Carrying Tubes and Flexible Hoses

In the case of tubes and hoses used to carry fluid, “AN Size” designates the outer diameter of the hose or tubing in sixteenth-inch increments. For example, an AN size -2 designates a tube OD of eighth-inch (2 x 1/16”), the tube OD of AN size -3 is 3/16” (3 x 1/16”). The AN size number is correctly displayed with a dash before the number.

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 Chart

Adapter Fittings

Also known as standard or union fittings, adapter fittings are commonly used to connect fuel line tubes and hoses to fuel pumps, fuel regulators, and fuel filters. Adapter fittings are available in SAE size configurations, but most high-end racing and performance fuel systems use those manufactured to AN standards. With AN versions, the fitting typically has a 37-degree 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, because 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. Use of The O-ring seals allow for greater sealing ability with high reliability of installation compared with a tapered thread that uses sealing compounds or Teflon tape.

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-degree 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. However, in high-pressure fuel system applications this method has proven more leak prone than the O-ring seal method used with straight thread fittings. Also, Teflon tape has the potential to leave debris in the fuel system. Which leads us to the port fittings.

Port Fittings

Port fittings function just like pipe fittings, but with the following distinctions. Port fittings conform to AN standards, typically per Military Standard MS33649. The cone end uses straight thread and a 37-degree 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 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 compared with 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. It should be noted that while manufacturers of high-performance fuel-delivery systems often specify port fittings for connections to fuel pumps and fuel-pump regulators, the connections to fuel-pressure gauges, via gauge ports, most often use pipe fittings. This is because the vast majority of the inline fuel-pressure gauges on the market use pipe fittings for connection.