Choosing the right AN fittings is one of the most critical decisions you will make in any high‑performance automotive project—whether you are building a race car, upgrading a street machine, or plumbing a custom fuel or oil system. AN fittings (Army‑Navy fittings) are engineered for reliability under extreme pressure and temperature, and they provide leak‑free connections that withstand vibration and thermal cycling. This guide covers everything you need to know: from the different types and materials to sizing, installation, and application‑specific considerations. By the end, you will be able to select the correct fittings with confidence and build a system that performs safely and durably.

What Are AN Fittings?

AN fittings are threaded connectors standardized under military specification MIL‑F‑5509 (now superseded by SAE AS4395). They feature a 37‑degree flare sealing surface and a straight (unflared) threaded nut. The seal is metal‑to‑metal, created when the flared end of a tube or hose is compressed against the fitting’s conical seat. This design provides excellent resistance to vibration, high pressure, and temperature extremes.

While often confused with JIC (Joint Industry Conference) fittings, AN fittings are dimensionally identical but manufactured to tighter tolerances and with stricter material specifications. For high‑performance automotive use, genuine AN fittings are preferred for critical systems like fuel lines and oil coolers.

AN fittings are available in a wide range of configurations—straight connectors, elbows, tees, bulkhead fittings, reducers, and more—to route hoses and tubes cleanly through tight engine bays.

AN Fitting Size and Dash Number System

AN fittings are sized by a dash number that represents the tube’s outside diameter (OD) in 1/16‑inch increments. For example, a –6 fitting has a nominal OD of 6/16 inch (3/8 inch). This system simplifies selection because the dash number matches the hose or tube size. Common sizes and their applications include:

  • –4 (1/4 inch OD): Used for small fuel lines, boost reference lines, and transmission cooler lines.
  • –6 (3/8 inch OD): Most popular for general fuel systems, oil cooler lines, and some power steering lines.
  • –8 (1/2 inch OD): Common for return lines, larger oil systems, and moderate flow fuel systems.
  • –10 (5/8 inch OD) and larger: Used for high‑flow fuel systems (e.g., carbureted high‑horsepower engines) or large coolant lines.

It is important to note that the dash number refers to the tube or hose OD, not the thread size. For example, a –6 AN fitting uses a 9/16‑18 UNF thread, while a –8 uses a 3/4‑16 UNF thread. Always match the fitting to the hose or tube you plan to use—mismatched sizes will leak or fail.

Materials and Their Compatibility

AN fittings are manufactured from several materials, each with distinct benefits and limitations. Choosing the wrong material can lead to galvanic corrosion, pressure failure, or premature wear.

Aluminum

Aluminum fittings are light, corrosion‑resistant, and inexpensive. They are ideal for most street and track applications where operating pressures do not exceed the material’s limits (typically up to 1,500 psi depending on size and design). Many aftermarket fuel and oil systems use blue‑ or red‑anodized aluminum fittings for easy identification. However, aluminum is softer than steel and can strip threads if over‑torqued, especially in high‑vibration environments.

Steel

Steel (usually zinc‑plated) fittings are stronger than aluminum and suitable for higher pressures, such as brake systems or high‑pressure fuel injection. They resist deformation under repeated assembly. The downside is weight and susceptibility to corrosion if the plating is scratched. For most automotive plumbing, steel fittings are used sparingly—often for bulkhead or adapter applications where strength is critical.

Stainless Steel

Stainless steel fittings combine the strength of steel with excellent corrosion resistance. They are the top choice for marine environments, turbo oil drain lines, or any system exposed to salt, moisture, or corrosive fluids. Stainless fittings are more expensive and harder to machine, but they offer the best longevity. When using stainless fittings with aluminum hose ends, be cautious of galvanic corrosion—use an anti‑seize compound or isolate the metals.

Brass

Brass fittings are occasionally used in low‑pressure applications (e.g., some carburetor fuel lines). They are easy to machine and corrosion‑resistant but are not recommended for high‑pressure systems because of their lower strength and tendency to crack under stress. Brass is more common in JIC plumbing than in true AN systems.

Types of AN Fittings

Beyond material and size, the fitting configuration determines how you route the hose. Below are the most common AN fitting types used in automotive builds.

Straight Fittings

Straight connectors join two hoses or a hose to a hard line in a linear path. They are the simplest and most space‑efficient option when clearance is not an issue.

Elbow Fittings (45° and 90°)

Elbow fittings change the direction of the hose. They are essential in tight engine compartments where a straight run is impossible. Use a 90° elbow to make a sharp turn, and a 45° elbow for a gentler bend. Swivel‑type elbows allow orientation after tightening, which simplifies installation.

T‑Fittings (Tees)

Tee fittings split a single feed into two branches or combine two lines into one. They are common in fuel systems (return lines) or oil cooler circuits. Ensure the tee is rated for the combined flow and pressure.

Bulkhead Fittings

Bulkhead fittings pass through a firewall, chassis member, or tank wall. They have a shoulder and nut on the outside and a threaded body on the inside. Use bulkheads when you need a clean, sealed pass‑through—for example, running a fuel line through a trunk floor or firewall.

Reducer and Adapter Fittings

Reducers step down from one dash size to another (e.g., –8 to –6). Adapters change thread types (e.g., NPT to AN) or connect components with different port sizes. Always use reducers near the component rather than in the middle of a hose to avoid flow restrictions.

Cap and Plug Fittings

Caps seal the end of an AN port or hose to prevent dirt ingress or leaks. Plugs close off unused ports on manifolds or components. Both are often supplied with O‑rings to ensure a positive seal.

Choosing the Right Hose and Fitting Combination

AN fittings work with specific hose types. The hose must be compatible with both the fitting style and the system fluid. Common hose options include:

Braided Stainless Steel Hose (PTFE Lined)

This is the gold standard for fuel and oil systems. PTFE (Teflon) inner lining is chemically inert and handles high temperatures and pressure. The stainless steel braid provides abrasion resistance and mechanical strength. Use PTFE fittings with a 37‑degree flare and a ferrule that compresses the braid. PTFE hoses are stiff and require careful routing—avoid sharp bends.

Rubber Push‑Lock Hose

Push‑lock (or barbed) hoses use a rubber inner tube reinforced with synthetic fiber. They connect to barbed AN fittings without clamps—simply push the hose over the barb. These hoses are flexible, easy to install, and ideal for low‑ to medium‑pressure applications such as oil cooler lines or power steering. However, they are not compatible with all fluids (check chemical resistance) and cannot handle the same pressures as PTFE‑lined hose.

Reusable Fittings

Reusable AN fittings (also called field‑assemblable) consist of a male body, a nut, and a compression sleeve (ferrule). They can be disassembled and reused on new hose ends, making them popular for custom builds. The hose is inserted through the nut and ferrule, flared if needed, and the nut tightens to compress the ferrule onto the hose. Always lubricate the threads and the ferrule to ensure a proper seal.

AN Fitting Installation Best Practices

Proper installation is as important as selecting the correct fitting. Follow these steps to achieve a reliable, leak‑free connection.

  1. Cut the hose squarely using a sharp saw or dedicated hose cutter. Deburr the inside and outside edges of metal‑braided hoses.
  2. Lubricate threads and seals with a small amount of thread sealant or Teflon tape on NPT threads only. On AN 37‑degree flare connections, do not apply Teflon tape to the flared seat—the metal‑to‑metal seal does the work. Lubricate the threads with a drop of oil or anti‑seize.
  3. Hand‑tighten first to align the fitting, then use a backup wrench on the hex to prevent twisting the hose. Tighten until the fitting bottoms out on the flare—further tightening may distort the seat. Many manufacturers specify a torque value (e.g., 10–15 ft‑lb for –6 aluminum fittings).
  4. Conduct a pressure test before final assembly. Pressurize the system with compressed air or fluid and inspect every connection for leaks. Re‑tighten if necessary, but avoid over‑torquing.
  5. Use the correct wrenches (crowsfoot or open‑end) to avoid rounding the hex. Some fittings have a specific hex size; using an adjustable wrench can damage the fitting.

Common Automotive Applications

Each system in your vehicle imposes unique demands on fittings. Here is a breakdown by application.

Fuel Systems

High‑pressure fuel injection (50–70 psi) requires AN fittings rated for at least 150 psi burst. PTFE‑lined hose with stainless steel braid is recommended. Use –6 or –8 for feed lines on moderate builds; –10 or larger for high‑flow ethanol or E85 systems. Avoid aluminum fittings in ethanol systems unless they are anodized—raw aluminum can corrode in ethanol blends.

Oil Systems (Lubrication and Cooling)

Engine oil systems operate up to ~80 psi hot and can reach temperatures over 250°F. Aluminum fittings are acceptable for most street applications. For turbo oil drain lines (gravity‑fed), a –10 or –12 hose is common to prevent restriction. Use a 45° or 90° swivel fitting at the turbo to avoid hose kinking.

Coolant Systems

Coolant (water/ethylene glycol) does not require high pressure but can cause galvanic corrosion between dissimilar metals. Stainless steel fittings are best for long‑term reliability in cooling circuits. Use –8 or –10 hoses for heater lines, and –12 or larger for radiator connections on high‑horsepower builds.

Brake Systems

Brake pressure can exceed 2,000 psi under hard braking. Only use steel or stainless steel fittings for brake lines. Never use aluminum fittings—they are not strong enough. Stainless steel braided brake hoses with AN fittings are common on performance cars, but ensure the fittings are DOT‑compliant if the vehicle is street‑driven.

Turbo Oil and Water Lines

Turbochargers require a reliable supply of oil and sometimes water. The oil feed line is usually –4 or –3 (e.g., 3/16 inch), while the drain is –10. Use high‑temperature PTFE hose. For water lines –6 or –8 is typical. Swivel fittings on both the turbo and the engine block ease installation.

Purchasing Considerations and Quality

Not all AN fittings are created equal. Look for fittings that meet SAE AS4395 standards. Reputable manufacturers include Aeroquip, Earl’s (Holley), Russell (Edelbrock), and Fragola. Avoid unbranded or budget fittings—they may have inconsistent thread tolerances or weak materials that lead to leaks.

When ordering, pay attention to the thread direction and seat type. Most AN fittings are male with a 37‑degree flare seal, but some components (like carburetors or fuel pumps) have NPT ports—you will need an NPT‑to‑AN adapter. Also note that some brands use a different sealing mechanism, such as O‑ring face seal (ORFS) for hydraulic applications.

For further reading, consider the following external resources (links open in a new tab):

Conclusion

Selecting the right AN fittings for your automotive project is not complicated when you understand the fundamentals: the dash size determines the flow capacity, the material must match the fluid and environment, and the fitting style should fit the physical routing demands. Always prioritize quality components from trusted manufacturers, and take the time to install them correctly. A well‑plumbed system using proper AN fittings will deliver reliable performance, resist leaks, and stand up to the harshest driving conditions. Whether you are building a daily driver, a track beast, or a show‑quality restoration, the right fittings are the foundation of a safe and functional vehicle.