Introduction to Custom AN Fitting Systems

Building a custom AN (Army-Navy) fitting system for your hot rod or classic car is one of the most rewarding upgrades you can make. It transforms the way your vehicle handles fuel, oil, and coolant delivery, often eliminating leaks that plague older rubber hoses and clamps. A properly designed AN system not only looks clean under the hood but also improves reliability and safety. Whether you are restoring a vintage muscle car or building a custom street machine, understanding how to design, assemble, and maintain AN fittings is essential for long-term success.

This guide covers everything from basic terminology to advanced assembly techniques, with an emphasis on practical knowledge you can apply in your own garage. By the end, you will have the confidence to create a leak-free, professional-grade system that matches your vehicle’s unique requirements.

What Are AN Fittings and Why Use Them?

AN fittings were originally developed for military aircraft, where reliability under extreme pressure and vibration was critical. The standard uses a 37-degree flare seat, which creates a metal-to-metal seal that is both strong and reusable. Unlike standard compression fittings or barbed hose connections, AN fittings provide a positive seal that resists loosening from engine heat and chassis flex.

In the automotive world, AN fittings are preferred for high-performance fuel systems, oil coolers, power steering, brake lines, and turbo oil feed/return lines. Their advantages include:

  • Leak-proof design – The 37-degree flare and precision threads ensure a tight seal without needing excessive tightening.
  • Reusability – You can disconnect and reconnect lines without damaging the seal, unlike crimped hose ends.
  • Flow capabilities – AN fittings have a large internal bore relative to their outer size, reducing flow restriction.
  • Vibration resistance – The threaded connection stays secure even in high-vibration environments.
  • Visual appeal – Available in anodized aluminum or stainless steel, they give your engine bay a clean, custom appearance.

“The 37-degree flare is the backbone of the AN system. It’s a proven military standard that has been adapted by racers and hot rodders for decades.” – Earl’s Performance Plumbing Tech Center

Planning Your System: Layout and Measurements

Before buying any fittings, spend time planning the routing of your lines. A well-designed system avoids sharp bends, reduces the number of fittings needed, and maintains accessibility for maintenance. Start by disconnecting the old lines and cleaning the area. Then use flexible measuring tape or string to trace the path we want the new lines to follow.

Key Considerations for Routing

  • Heat sources – Keep lines away from exhaust manifolds, headers, and catalytic converters. Use heat sleeves or routing around cool zones if necessary.
  • Moving parts – Allow clearance from suspension components, steering shafts, and fan belts.
  • Serviceability – Position fittings so they can be easily reached with a wrench for future disconnection.
  • Drainage – Slope lines slightly downward to prevent air pockets, especially in fuel systems.

Create a simple sketch of the engine bay and mark the locations of the pump, filter, cooler, regulator, and tank. Note the direction of flow and any existing threaded ports. This diagram will serve as your shopping list when ordering fittings and hose.

Selecting the Right Hose and Tubing

You have two primary choices for AN system lines: braided stainless steel hose or hard aluminum/steel tubing. Each has its strengths.

Braided Hose (Push-Lok or Crimp-Style)

Braided hose is flexible, easy to route around obstacles, and forgiving of minor alignment errors. It is the most common choice for hot rods and street machines. Inner linings can be rubber (for fuel and oil) or PTFE/Teflon (for high-heat applications like turbo oil feeds). The outer braid protects against abrasion and cuts.

Hard Tubing (Aluminum or Stainless Steel)

Hard tubing provides a rigid, permanent installation that looks extremely clean. It is ideal for straight runs between components mounted in fixed positions, such as from a fuel filter to the carburetor or from the engine block to an oil cooler. However, tubing requires precise bending and flaring, and you must ensure that all bends are gradual to avoid restricting flow.

For most hobbyists, a combination of both works best: hard tubing for visible straight sections and braided hose for flexible connections near the engine or where vibration is high.

Choosing AN Fittings: Sizes, Materials, and Types

AN fittings come in standard dash sizes: -3, -4, -5, -6, -8, -10, -12, -16, and larger. The number indicates the outer diameter of the tube in sixteenths of an inch. For example, a -6 AN fitting fits 3/8-inch OD tube. However, the hose inner diameter is slightly smaller. Common applications:

  • -4 AN – Small lines like boost reference, nitrous, or small oil feeds.
  • -6 AN – Standard fuel lines for most V8 engines (roughly 3/8″ ID).
  • -8 AN – Large fuel lines for high-horsepower EFI setups or oil cooler lines.
  • -10 AN and larger – Main oil gallery feeds, large coolant lines, or transmission coolers.

Fittings are typically made from 6061-T6 aluminum or 303 stainless steel. Aluminum is lighter and cheaper, but stainless is stronger and more corrosion-resistant. For high-temperature or high-pressure applications like brake lines, use stainless steel or a DOT-approved equivalent.

Common Fitting Types You Will Need

  • Straight adapter (AN to NPT) – Connects the AN hose end to a component with a national pipe taper thread (e.g., fuel pump or carburetor).
  • 90-degree and 45-degree swivel – Allows the hose to bend at an angle while keeping the fitting alignment correct.
  • Union or reducer – Joins two lines of the same or different sizes.
  • Bulkhead fitting – Passes through a panel or firewall securely.
  • Hose end – The part that attaches to the hose. Available in reusable (field-serviceable) or crimp-only styles.

Always match the hose end to the hose you intend to use. Push-lok hose requires push-lok fittings, while standard braided hose uses barbed or socket-style ends. Refer to manufacturer compatibility charts before purchasing.

Tools and Materials Needed

Having the right tools makes assembly easier and prevents damage to components. Here is a comprehensive list:

  • AN wrenches (line wrenches) – use crow’s foot or flare nut wrenches in the correct dash sizes to avoid rounding fitting hexes.
  • Hose cutter (ratcheting shears work best) – a clean square cut is essential for a good seal.
  • Flaring tool – if using hard tubing, a 37-degree flaring kit is required (do not use a 45-degree SAE flare tool).
  • Deburring tool – to remove sharp edges inside the tube after cutting.
  • Thread sealant or Teflon tape – paste-style sealant is preferred for NPT threads; use only on the pipe threads, never on the AN flare seat.
  • Measuring tape, marker, and notepad – for layout and documentation.
  • Safety glasses and gloves – because cutting stainless braid can cause flying particles.

Step-by-Step Assembly Process

Now we will walk through assembling a typical AN system using braided hose and reusable ends. If you are using hard tubing, the flaring procedure varies slightly, but the connection principles remain the same.

1. Cut the Hose

Wrap the hose tightly with electrical tape where you plan to cut. This prevents the stainless steel braid from fraying. Use a high-quality hose cutter or an angle grinder with a cutoff wheel for a clean cut. Remove the tape and inspect the cut end – it must be straight and square.

2. Insert the Socket and Nipple

For reusable hose ends, push the socket onto the hose, then insert the nipple. Lubricate the inside of the hose with a drop of light oil or assembly lube to help the nipple slide in. Use a vise with soft jaws to hold the socket while you drive the nipple home. Some reusable fittings require a special tool to press the two parts together.

3. Attach Fittings to Components

Thread the adapter into the component port (e.g., fuel pump outlet) using thread sealant on the NPT threads. Hand-tighten then use a wrench to snug – do not over-torque aluminum fittings. The torque spec for -6 AN into aluminum is around 20-25 ft-lbs; for stainless, 30-35 ft-lbs.

4. Connect the Hose

Install the hose end onto the adapter fitting. The flare of the adapter must seat against the flare of the hose end. Use two wrenches to tighten: one on the adapter to hold it steady, one on the hose end nut. Tighten to manufacturer specifications, typically 1/6 to 1/4 turn beyond hand-tight.

5. Route and Secure the Lines

Lay the assembled lines into the engine bay, using nylon clamps or aluminum brackets every 12-18 inches to prevent chafing. Avoid overtightening the clamps as this can squeeze the hose and reduce flow. Leave a small amount of slack at each end to accommodate engine movement.

Testing for Leaks

Before starting the engine, pressurize the system to check for leaks. The safest method is to use low-pressure air (5-10 PSI) while blocking the outlets. Listen for hissing and apply soapy water to each connection – bubbles indicate a leak. Tighten as needed, but remember that overtightening can damage the flare seat.

For fuel systems, you can also pressurize with the fuel pump by cycling the key without starting (if your car has an electric pump). Check every fitting from the tank to the carburetor or injectors. Once the system holds pressure, start the engine and recheck while it runs. Allow the engine to reach operating temperature to verify that thermal expansion does not cause leaks.

Common Mistakes to Avoid

  • Using Teflon tape on flare threads – Never apply sealant to the 37-degree flare mating surface. It can prevent a metal-to-metal seal and cause loosening.
  • Cross-threading fittings – Always start fittings by hand to ensure alignment, then use a wrench.
  • Sharp bends – The minimum bend radius for braided hose is about 5-6 times the hose outer diameter. Tighter bends restrict flow and may fatigue the braid.
  • Mixing AN and SAE/JIC fittings – SAE 45-degree flares look similar but will not seal properly. Always use JIC 37-degree components labelled AN.
  • Overtightening – AN fittings seal best when tightened to spec, not as tight as possible. Too much torque can distort the flare or crack the adapter.

Maintenance and Long-Term Care

A properly assembled AN system requires minimal upkeep, but regular checks will extend its life. Inspect hose outer braid for fraying or corrosion every oil change. Look for weeping fluid at the junctions, especially after the first few heat cycles. If you notice a leak, do not just tighten blindly – inspect the flare seat for scratches or debris. A damaged flare can sometimes be re-dressed with a fine file, but replacement is safer.

For hard tubing systems, check that clamps have not loosened and that the tubing has not shifted due to vibration. Replace any hose that feels hard or cracked; braided hose typically lasts 5–10 years depending on exposure to heat and UV.

When sourcing fittings, stick to reputable brands such as Summit Racing, Jegs, Earl’s Performance, or AN Plumbing. Their technical support teams can help with sizing and compatibility. Online forums and YouTube channels dedicated to hot rodding and track cars are also invaluable for real-world installation tips.

Conclusion

Creating a custom AN fitting system for your hot rod or classic car is a project that blends mechanical skill with thoughtful design. By understanding the fundamentals of AN fittings, carefully planning your layout, selecting quality materials, and assembling with precision, you can build a system that is both functional and visually impressive. Take your time during the planning phase, invest in good tools, and do not rush the testing process. A leak-free, properly routed AN system will serve your ride reliably for years and give you the confidence to enjoy every mile.