AN (Army-Navy) fittings are the gold standard for secure, leak-free fluid connections in high-performance automotive, aerospace, marine, and industrial systems. Whether you are plumbing a race car fuel system, building a hydraulic press, or maintaining an aircraft, the pressure rating of each fitting determines the safety and reliability of the entire system. Understanding these ratings is not just a technical exercise — it is the difference between a system that performs flawlessly and one that fails under stress. This guide breaks down what AN fitting pressure ratings mean, how they vary by material and design, and how to select the right fitting for your application.

What Are AN Fittings?

AN fittings were originally developed by the U.S. military (Army-Navy) during World War II to create a standardized, interchangeable connector for fluid systems. Today, they are governed by SAE (Society of Automotive Engineers) specifications, primarily AS4395 and AS4841/2. AN fittings use a 37-degree flare seat and a threaded nut to create a metal-to-metal seal — no O-rings or gaskets required. This design provides exceptional vibration resistance and temperature tolerance, making them ideal for demanding environments.

Common materials include:

  • Aluminum 6061-T6 — lightweight, corrosion-resistant, good for low to moderate pressures.
  • Carbon steel — stronger, suitable for high-pressure systems, but heavier and prone to rust if uncoated.
  • Stainless steel (304 or 316) — highest strength and corrosion resistance, used in marine and chemical applications.

AN sizes range from -2 (1/8″ tube OD) to -32 (2″ tube OD) and beyond. Each size has a corresponding pressure rating that decreases as the fitting gets larger — a fundamental relationship driven by hoop stress in the material.

Understanding Pressure Ratings

Pressure ratings for AN fittings are not arbitrary numbers. They are determined by rigorous testing per SAE J514 and similar standards. Two critical values define any fitting’s pressure capability:

  • Working pressure (WP) — the maximum sustained pressure a fitting can handle under normal operating conditions. This is the number you use for design.
  • Burst pressure — the pressure at which the fitting is expected to fail. Burst pressure is typically 3 to 4 times the working pressure, providing a safety margin.

Pressure ratings are typically expressed in psi (pounds per square inch). For example, a -6 aluminum AN fitting may be rated at 1,500 psi working pressure and 6,000 psi burst.

It is critical to remember that the fitting is only as strong as its weakest link — the hose, the tube end, and the female port all have their own ratings. Always verify the entire assembly’s pressure capacity.

Factors Affecting Pressure Ratings

Material

Material strength directly impacts pressure capacity. Aluminum fittings are lighter and cheaper but have lower tensile strength than steel or stainless. Steel fittings can handle 2 to 3 times the working pressure of an equivalent aluminum fitting.

Fitting Size

Pressure rating decreases as fitting size increases. A -4 (1/4″) aluminum fitting might handle 3,000 psi working pressure, while a -16 (1″) version of the same type may be rated for only 500 psi. This is due to increased surface area and hoop stress.

Temperature

High temperatures reduce the yield strength of metals. Many manufacturers derate pressure ratings above 200°F (93°C). For example, a steel fitting rated for 6,000 psi at room temperature may drop to 4,500 psi at 400°F.

Fluid Type

Some fluids (e.g., hydraulic oils, fuels, acids) can cause chemical attack or stress corrosion cracking. Stainless steel is preferred for corrosive fluids, while aluminum is compatible with most petroleum-based products but not strong acids.

Thread and Seal Design

AN fittings use straight threads with a 37-degree flare seal. The nut and body threads carry the load; any galling, cross-threading, or overtightening can reduce the effective pressure rating. Using anti-seize compound on stainless steel fittings is recommended to prevent galling.

Types of AN Fittings and Their Pressure Ratings

Below is a guide to typical working pressure ranges for common AN fitting materials. Always consult the manufacturer’s specification sheet for exact numbers — these are general reference values.

Typical Working Pressure Ranges for AN Fittings (psi)
Fitting Material Size -4 (1/4″) Size -6 (3/8″) Size -8 (1/2″) Size -10 (5/8″) Size -12 (3/4″)
Aluminum 6061-T6 3,000 2,500 2,000 1,500 1,200
Carbon steel 6,000 5,000 4,000 3,500 3,000
Stainless steel 304/316 8,000 6,500 5,500 4,500 3,500

Note: These values assume standard temperature (70°F) and non-pulsating pressure. For pulsating or shock loads, derate by 50% or more. Always include a safety factor — typically 1.5 to 2 times the maximum system pressure.

Specialty AN Fittings

Some applications require fittings with higher ratings. Hydraulic swivel fittings, bulkhead adapters, and reducer bushings often have lower pressure capacities than straight connectors due to additional stress risers. Always check individual part listings.

How to Choose the Right AN Fitting

Selecting the appropriate AN fitting involves more than matching size. Follow these steps to ensure a safe, long-lasting connection:

  1. Determine maximum system pressure — including spikes and pressure surges. Do not guess; use a pressure gauge or refer to pump specifications.
  2. Select a fitting material that can handle that pressure with a safety margin of at least 1.5 times. If your system runs at 2,000 psi, choose a fitting rated for at least 3,000 psi working pressure.
  3. Consider the fluid and environment. Use stainless steel for corrosive fluids or exposed marine use. Aluminum is fine for fuel, oil, and moderate temperatures.
  4. Check temperature range. If the fluid or ambient temperature exceeds 200°F, apply the manufacturer’s temperature derating factor.
  5. Match the fitting type to your system layout — long-radius elbows reduce stress, while straight fittings are strongest. Avoid mixing materials (e.g., steel nut on aluminum body) unless the manufacturer approves.
  6. Verify compatibility with hose or tube. The pressure rating of the hose end or tube flare must equal or exceed the fitting rating. A common mistake is using a high-pressure fitting with a low-pressure hose.

Best Practices for Installation and Maintenance

Even the highest-rated AN fitting will fail if improperly installed. Adhere to these guidelines:

  • Use the correct torque. Over-tightening can crack the aluminum flare or strip threads. Most manufacturers provide a torque chart; for example, a -6 aluminum nut typically requires 10–12 ft-lbs, while steel may need 15–20 ft-lbs. Use a torque wrench.
  • Lubricate threads on stainless steel fittings to prevent galling. Teflon-based or molybdenum disulfide anti-seize compounds are common.
  • Never use Teflon tape on the flare seat. Tape can interfere with the metal-to-metal seal and cause leaks. If a sealant is needed, use a drop of thread-locker on the nut threads — not on the flare.
  • Inspect fittings regularly for cracks, distortion, corrosion, or thread wear. Replace any fitting that shows damage — it may have lost its rated pressure capacity.
  • Support heavy assemblies with brackets or clamps. Vibration fatigue is a leading cause of fitting failure, especially in engines and pumps.
  • Pressure test the completed system before putting it into service. Use a test pressure of 1.5 to 2 times the working pressure (but never exceed the burst rating of any component).

Common Myths About AN Fitting Pressure Ratings

Misinformation can lead to dangerous design choices. Here are three myths debunked:

  • Myth: “AN fittings are all rated the same.” False. Material, size, and design (straight vs. elbow) cause wide variations. Always read the label.
  • Myth: “The burst pressure is the working pressure.” No. Burst is a one-time limit; working pressure is for sustained use. Exceeding working pressure repeatedly can cause fatigue failure.
  • Myth: “Stainless steel is always stronger than aluminum.” Generally true, but small-diameter aluminum fittings can have higher working pressures than large-diameter stainless ones. Size matters more than material in many cases.

Where to Find Reliable Pressure Rating Data

Always obtain manufacturer-specific data rather than relying on generic tables. Reputable suppliers publish detailed specification sheets. See for example:

Conclusion

Pressure ratings are not optional data — they are critical safety parameters. By understanding how material, size, temperature, and design affect AN fitting performance, you can select components that provide reliable, leak-free service for years. Always cross-reference manufacturer specifications, build in a safety margin, and follow proper installation practices. When in doubt, choose a fitting rated higher than your system’s maximum pressure — it is far better to overspec than to risk a catastrophic failure. Properly rated AN fittings are the backbone of any high-performance fluid system, and knowledge of their ratings keeps your projects safe and efficient.