Upgrading from original equipment manufacturer (OEM) fittings to aftermarket AN fittings is a common step for enthusiasts looking to increase performance, improve reliability, or add a custom aesthetic to their vehicle’s fluid systems. Whether you are building a track car, a custom street machine, or simply replacing aging components, understanding the differences and following a systematic approach ensures a successful transition that meets your goals without compromising safety.

What Are AN Fittings and Why Upgrade?

AN stands for Army-Navy, a standard originally developed for military aircraft. These fittings use a 37-degree flare and a threaded nut to create a seal that is both strong and leak-resistant. Aftermarket AN fittings are widely used in racing, off-roading, and high-performance automotive applications because they offer superior material quality, precise sizing, and the ability to handle higher pressures and temperatures compared to many OEM fittings.

OEM fittings are designed for mass production and cost efficiency, often using nylon, plastic, or lower-grade metals. They can work well for normal driving conditions but may become brittle, corrode, or fail under extreme heat, vibration, or pressure. Switching to aftermarket AN fittings provides:

  • Higher burst pressure ratings – especially important for fuel, oil, and brake systems.
  • Better corrosion resistance – with options like anodized aluminum, stainless steel, or nickel-plated brass.
  • Reusability – many AN fittings can be disassembled and reused with new hose ends.
  • Wider range of sizes and configurations – from -3AN (small) up to -20AN and beyond, plus swivel, bulkhead, and adapter styles.
  • Custom fit and appearance – choose colors, angles, and hose routing to match your build.

OEM vs. Aftermarket AN Fittings: Key Differences

To make an informed transition, it helps to compare the two fitting families in detail.

Materials and Construction

OEM fittings are often made from low-carbon steel, brass, or plastic with rubber O-rings or crush washers for sealing. Aftermarket AN fittings are machined from 6061 aluminum, 304 stainless steel, or high-strength steel with precision threads and a 37-degree flare sealing surface. The absence of O-rings (in most AN systems) reduces potential failure points.

Sealing Mechanism

OEM systems may use flareless compression sleeves, push-lock barbs, or threaded connections with gaskets. AN fittings rely on a precision-machined 37° flare that mates with a matching flare on the hose end or adapter. When the nut is tightened, metal-to-metal contact creates a seal that is resistant to vibration and thermal cycling.

Size Standards

OEM thread sizes are often metric or specific to a manufacturer (e.g., GM, Ford, Mopar). AN sizes are based on dash numbers: -4AN has a 1/4” tube OD, -6AN is 3/8”, -8AN is 1/2”, and so on. Adapters are available to connect AN to NPT, BSPP, JIC, or metric threads. Always verify thread pitch and sealing type before purchasing adapters.

Pressure and Temperature Ratings

Aftermarket AN fittings typically exceed the ratings of most OEM components. For example, a standard -6AN aluminum fitting might have a working pressure of 1,500 psi and a burst pressure over 6,000 psi. Stainless steel versions can handle even more. OEM plastic quick-connects may fail at pressures as low as 200 psi when heated.

Planning Your Transition: A Step-by-Step Guide

Moving from OEM to aftermarket AN fittings is not a simple swap. Each system (fuel, oil, coolant, brake, power steering) has unique requirements. Follow these steps to ensure a safe and effective conversion.

Step 1: Assess Your Vehicle’s Needs and System

Decide which systems you want to upgrade. Start with the most critical or performance-sensitive ones: fuel delivery, oil cooler lines, or brake lines. For each system, consider:

  • Fluid type and temperature – some hoses and fittings are not compatible with certain fluids (e.g., E85, brake fluid, synthetic oils).
  • Operating pressure – fuel injection systems can see 60-80 psi in return-style systems, while diesel high-pressure lines can exceed 30,000 psi (requiring specialized fittings, not standard AN).
  • Hose routing – tight spaces may require 45° or 90° swivel fittings to avoid kinks.
  • Environmental exposure – under-hood heat, road debris, and salt can degrade certain materials.

Document current OEM fitting sizes and thread types. A thread pitch gauge and a digital caliper are invaluable for this step.

Step 2: Identify Compatible Aftermarket Fittings and Adapters

Once you know the OEM threads, find the corresponding AN adapter. Common examples:

  • GM power steering pump uses O-ring to -6AN adapters.
  • LS engine fuel rails often use M12x1.5 metric ports that accept -6AN ORB fittings.
  • Radiator drain cocks and heater cores may require 1/2” NPT to -6AN adapters.

External link: Summit Racing offers a comprehensive fitting selection with search filters by thread size and style. For technical diagrams, see AN Fittings Direct.

Step 3: Choose the Right Hose and Hose Ends

AN fittings require compatible hose. The most common types are:

  • Rubber push-lock hose – easy to install without special tools, but less durable under high heat. Suitable for lower-pressure applications like oil coolers.
  • PTFE (Teflon) lined hose – excellent chemical resistance, low friction, and can handle high temperatures. Used for fuel injection and brake systems. Requires a specific hose end with a reusable mandrel.
  • Stainless steel braided hose (e.g., Aeroquip AQP or PTFE) – the most durable and high-performance option. Requires a vise and wrenches to assemble the socket and nipple.

Select hose ends that match the hose type. For reusable ends, ensure the socket and nipple are designed for the hose’s outer diameter and braid type.

Step 4: Gather Necessary Tools and Safety Equipment

Having the right tools prevents damage to fittings and assures proper assembly. Essential tools include:

  • AN wrenches (crows foot or flare nut wrenches) – never use standard open-end wrenches on AN nuts, as they can round the flats.
  • Hose cutter (tubing cutter for PTFE, sharp utility knife for rubber).
  • Deburring tool for PTFE hose.
  • Thread chaser set to clean OEM ports.
  • Torque wrench – many AN fittings have specified torque values (e.g., 10-15 ft-lbs for -6AN aluminum).
  • Safety glasses and nitrile gloves. Ensure the work area is well-ventilated and free of ignition sources when working with fuel lines.

Step 5: Drain Fluids and Remove OEM Fittings

Always relieve system pressure and drain fluids into an approved container. For fuel systems, disconnect the battery and allow the engine to cool. Remove OEM fittings carefully using the correct socket or wrench. If a fitting is corroded, apply penetrating oil and allow it to soak. Avoid using excessive force that could damage the mating component (e.g., an aluminum radiator tank).

Inspect the mating surfaces for burrs, scratches, or debris. Clean threads with a solvent and chase them if necessary. For flare-seal connections, ensure the sealing surface is smooth and free of pitting.

Step 6: Install Aftermarket AN Fittings and Hoses

Assemble the hose ends according to the manufacturer’s instructions. For reusable ends:

  1. Cut the hose squarely and remove any loose braid.
  2. Lubricate the inside of the hose (with silicone spray or the supplied lubricant).
  3. Insert the nipple into the hose and rotate until fully seated.
  4. Slide the socket over the hose and thread it onto the nipple. Use a vise and wrenches to tighten to the recommended marks.

When installing the assembled hose to the vehicle, start the nut by hand to avoid cross-threading. Use an AN wrench to tighten until the flats align. Do not over-torque aluminum fittings – they can crack.

Step 7: Pressure Test and Leak Check

After installation, pressurize the system slowly. For fuel lines, you can use a hand-held pressure pump or cycle the ignition key to build pressure. Inspect every connection with a mirror and a clean rag. Look for weeping, dripping, or discoloration. For brake systems, bleed the brakes and perform a firm pedal test before driving.

Run the engine and bring the system up to operating temperature. Check for leaks again after thermal expansion. Tighten any loose connections incrementally – over-tightening can deform the sealing surfaces.

Common Mistakes to Avoid

  • Mixing tread types – never thread a NPT fitting into a JIC port, or vice versa, without an adapter. The taper angles differ.
  • Using Teflon tape on AN threads – AN fittings seal on the flare, not the threads. Tape can cause over-tightening and contaminate the system. Use only on NPT adapter threads.
  • Routing hoses near exhaust without adequate shielding – braided stainless can still degrade with prolonged heat.
  • Overtightening – can crack fittings, strip threads, or deform the flare. Follow torque specs.
  • Assuming all -6AN fittings are identical – different brands may have slight variations in hex size or thread depth. Stick with one brand per system for consistency.

Benefits Beyond Performance

While performance gains are often cited, switching to aftermarket AN fittings also improves serviceability. Many AN fittings can be disassembled without cutting the hose, allowing for future modifications or repairs. The standardized sizing makes it easy to add sensors, gauges, or remote filters. Additionally, the professional appearance of colored anodized fittings and braided hose enhances engine bay aesthetics.

Conclusion

Transitioning from OEM to aftermarket AN fittings is a rewarding upgrade that requires careful planning, accurate measurement, and adherence to proper assembly techniques. By understanding the differences in materials, sealing methods, and sizing, you can choose the right components for your specific application. Follow a methodical process: assess needs, identify adapters, select compatible hose and ends, use the correct tools, and always pressure-test before driving. With patience and attention to detail, you’ll enjoy a system that is more durable, easier to maintain, and ready for higher performance demands.

For further reading on AN fitting specifications and torque values, consult AN Fittings Direct’s guide or Summit Racing’s fitting cross-reference. Always prioritize safety and when in doubt, seek advice from a professional fabricator or a knowledgeable shop.