Understanding AN Fittings and Their Role in Automotive Fuel Systems

Selecting the correct AN fitting size for your automotive fuel system is a decision that directly impacts engine performance, reliability, and safety. Whether you are building a custom fuel system for a high‑horsepower race car or upgrading a daily driver, the right connection ensures leak‑free operation, proper fuel delivery, and long component life. AN fittings, originally developed for aerospace applications, have become the standard in performance and custom automotive plumbing because of their precision, durability, and wide availability. This guide explains AN sizing, helps you calculate flow needs, and provides practical advice for choosing and installing the right fittings for your project.

What Are AN Fittings and How Does the Sizing System Work?

The term “AN” stands for “Army‑Navy” or “Aerospace National,” a system standardized by the U.S. military. AN fittings use a dash number (‑4, ‑6, ‑8, ‑10, etc.) that corresponds to the tubing’s outer diameter in sixteenths of an inch. For example, a ‑6 AN fitting fits tubing with a 6/16″ outside diameter (3/8″). However, the inner diameter (ID) of the hose or tube determines the actual flow capacity. In practice:

  • AN‑4: 1/4″ ID (approx. 0.210″ actual hose ID) – used for small‑diameter, low‑pressure lines.
  • AN‑6: 3/8″ ID (approx. 0.320″ actual hose ID) – the most common size for fuel feed lines on engines up to 600 hp.
  • AN‑8: 1/2″ ID (approx. 0.450″ actual hose ID) – suitable for high‑volume fuel delivery on engines over 600 hp, or for return lines and fuel pumps.
  • AN‑10: 5/8″ ID (approx. 0.570″ actual hose ID) – used in extreme power applications, supercharged or turbocharged builds, and large‑diameter tank outlets.

Note that hose inner diameters are slightly smaller than the nominal size due to wall thickness. Always check the manufacturer’s flow data for exact capacity.

Dash‑Number to Inch Conversion

  • AN‑4 = 4/16″ = 1/4″ OD
  • AN‑6 = 6/16″ = 3/8″ OD
  • AN‑8 = 8/16″ = 1/2″ OD
  • AN‑10 = 10/16″ = 5/8″ OD
  • AN‑12 = 12/16″ = 3/4″ OD

Factors That Determine the Right AN Fitting Size for Your Fuel System

Choosing a size based solely on engine horsepower or guessing “bigger is better” can lead to problems – oversized lines hurt fuel pressure regulation and fuel atomization, while undersized lines starve the engine. Consider these four factors:

1. Fuel Flow Requirements

Calculate the fuel volume your engine needs at maximum power. A naturally aspirated gasoline engine requires roughly 0.5 lb of fuel per horsepower per hour. For example, a 500 hp engine needs about 250 lb/hr of fuel. At 6.25 lb/gal for gasoline, that’s 40 gal/hr or 67 liters/hr. To this, add a safety margin (30–50%) for the fuel pump’s recirculation, regulator bypass, and future upgrades. High‑flow injection systems, forced induction, and E85 (ethanol) require 20–30% more volume. Use this table as a rough guide (based on typical fuel pump flow at 43 psi):

  • AN‑4: Supports up to ~300 hp (fuel feed for small engines, auxiliary or return lines).
  • AN‑6: Supports 300–600 hp (standard for most performance fuel systems).
  • AN‑8: Supports 600–1,000+ hp (common in serious race cars, twin‑turbo setups, or when running E85).
  • AN‑10: Supports 1,000+ hp (used for main feed lines in extreme builds or long line runs).

2. System Pressure and Fuel Type

Higher fuel pressures (e.g., 58 psi for some returnless systems vs. 43 psi for typical return systems) reduce the effective flow rate of a given hose size because the fuel pump’s flow curve drops with rising pressure. If you use high pressure, consider stepping up one size to maintain volume. Also, E85 and methanol have lower energy density per gallon and higher corrosive potential – they often require 30% more volume and compatible materials (stainless steel or PTFE inner liner).

3. Line Length and Routing

Long fuel lines create friction and pressure drop. For runs longer than 15 feet from tank to engine (common in rear‑tank setups), going one AN size larger than the pump outlet is a good practice. For example, if the pump has a ‑8 AN outlet, use ‑10 AN line for the main feed, or consider running a ‑8 feed and a separate return line of equal size to reduce back‑pressure.

4. Existing Component Compatibility

Match fitting sizes to the pump, regulator, filter, and fuel rail ports. Many high‑performance pumps come with ‑6 AN or ‑8 AN inlet and outlet ports. If you mix sizes, use reducers or adapters – but avoid excessive adapters that add leak points. It’s cleaner and safer to redesign the system to use one consistent size for all components.

Common AN Fitting Sizes for Different System Parts

Fuel Pump Inlet (Suction Side)

The suction side of the pump should be one size larger than the outlet to avoid cavitation. For example, a pump with a ‑8 AN outlet should have a ‑10 AN inlet. Many pumps even require a –12 AN or larger inlet hose. Always follow the pump manufacturer’s recommendations.

Fuel Feed Line (Pressure Side)

From the pump to the fuel rail or carburetor: use the size that matches your flow requirements (see table above). For EFI engines with return styles, the feed line is typically ‑6 or ‑8. For carbureted engines, a single ‑6 line is often sufficient up to 500 hp.

Return Line

For return‑style EFI systems, the return line should be the same size as the feed line to prevent back‑pressure that upsets the regulator. Some builders use one size smaller (e.g., ‑6 return on a ‑8 feed) on low‑pressure systems, but on high‑flow builds equal sizing is safer.

Vent and Breather Lines

Fuel tank vents and breather lines typically use ‑4 or ‑6 AN to allow air displacement without restricting fuel flow. Oversized vents can cause vapor lock, so stay close to the manufacturer’s recommendation.

Hose and Fitting Materials: What to Consider

Rubber Hose (Nitrile or Neoprene)

Suitable for low‑pressure applications (below 50 psi). It’s flexible and affordable but not compatible with all fuels – ethanol blends can degrade standard rubber. Use only hose labeled “EFI compatible” for fuel systems.

Stainless Steel Braided Hose (PTFE Lined)

The gold standard for high‑performance fuel systems. PTFE (Teflon) liner is chemically inert, handles high pressure (up to 1,000+ psi), and resists ethanol, methanol, and diesel. It’s more expensive and stiffer than rubber, but offers superior durability. Use with AL (anti‑chafe) or AQ (XQ‑style) fittings for best results.

Nylon/Braid Reinforced (E.g., Aeroquip AQP)

High temperature and abrasion resistance, compatible with all fuels, moderate cost. A good middle ground between rubber and PTFE.

Installation Tips for AN Fittings

Proper installation prevents leaks and ensures long‑term reliability.

  • Use correct wrenches: Always use an AN flare‑nut wrench or a line wrench (not an open‑end wrench) to avoid rounding the fitting flats. Torque to manufacturer specs – typically 15–25 ft‑lb for –6 AN, but always verify.
  • Lubricate threads: A small amount of oil or PTFE paste on the threads (not the flare) reduces galling on aluminum fittings. Never use Teflon tape on AN flare fittings – it can shred and clog fuel jets or injectors.
  • Check the flare angle: AN fittings use a 37‑degree flare. Do not mix 37‑degree AN fittings with 45‑degree JIC or inverted flare (automotive standard) – they are not interchangeable despite looking similar.
  • Support the lines: Use nylon or stainless steel clamps every 12–18 inches to prevent vibration fatigue. Allow a service loop at each end to avoid stress on the fitting.
  • Use a thread sealant on NPT adapters: When transitioning from AN to NPT (national pipe thread), use a liquid thread sealant (Loctite 567 or similar) on the NPT threads only. The AN side relies on the metal‑to‑metal flare seal.

Common Mistakes and How to Avoid Them

  • Oversizing the fuel line: A line too large can cause poor fuel pressure regulation and slower response in return‑style systems. Stick to the size that meets your flow needs – bigger is not always better.
  • Mixing dash numbers with hard line tubing: If you use hard line (e.g., 3/8″ steel line), ensure the AN adapter matches the outside diameter. Many 3/8″ hard lines require a ‑6 AN adapter, but always measure OD accurately.
  • Ignoring ethanol compatibility: Standard automotive rubber hoses and some low‑cost fittings are not rated for E85. Use PTFE hose and 300‑series stainless steel or anodized aluminum fittings for ethanol systems.
  • Not accounting for altitude or turbocharging: Forced induction and high altitude reduce fuel pump flow. Add an extra 10–20% margin when choosing line size for turbo/supercharged engines.
  • Over‑tightening: AN fittings seal by the interference fit of the 37‑degree flare, not by excessive torque. Over‑tightening deforms the threads or cracks the fitting. Use a torque wrench where possible.

External Resources and Further Reading

For detailed flow data and technical specifications, refer to these trusted sources:

Conclusion

Choosing the correct AN fitting size is a critical step in building a safe, efficient, and reliable automotive fuel system. By understanding the dash‑number system, calculating your engine’s actual fuel flow needs, and selecting compatible materials, you can avoid common pitfalls and optimize performance. Always consult manufacturer flow charts for your specific pump and regulator, and don’t hesitate to step up one size for long lines or aggressive fuel blends. With proper installation and the right fittings, your fuel system will deliver the fuel your engine demands for years to come.