Designing and building a custom AN fitting system for your car’s engine bay is one of the most satisfying upgrades you can make. It transforms a tangle of rubber hoses and barbed fittings into a clean, professional-looking fluid transfer network that not only looks the part but also delivers superior performance, durability, and reliability. Whether you are assembling a track-focused race car, a high-horsepower street machine, or simply refreshing a daily driver’s aging plumbing, understanding how to select, route, and install AN fittings correctly is essential. This guide walks you through every stage of the process—from understanding the basics of AN standards to planning your layout, choosing components, and performing a leak-free installation.

What Are AN Fittings and Why Use Them?

AN stands for Army-Navy, a specification originally developed for military aircraft and later adopted by the automotive and motorsport industries. The AN standard defines thread size, flare angle (37°), and hose-end dimensions, ensuring that components from different manufacturers can be interchanged reliably. Unlike standard pipe threads or compression fittings, AN fittings use a flared connection that creates a metal-to-metal seal—no O-rings, gaskets, or sealants required. This design allows them to handle extreme pressures, high temperatures, and aggressive fluids without leaking.

For engine bay applications, AN fittings offer several advantages: they are reusable, easy to disconnect and reconnect, and available in a wide range of configurations (straight, 45°, 90°, bulkhead, swivel, and more). They also give you full control over hose routing, enabling you to tuck lines neatly away from heat sources and moving parts. For these reasons, AN fittings are the standard in professional racing, custom fabrication, and high-performance street builds.

Planning Your Custom System

Before you order a single fitting, take time to plan your system on paper (or in a CAD sketch). A well-thought-out layout saves money, reduces frustration, and ensures a clean final result. Consider these critical factors:

  • Fluid type: Different fluids have different chemical and temperature properties. Oil, fuel, coolant, power steering fluid, and brake fluid each require compatible hose materials and fitting styles. For example, fuel systems require ethanol-resistant hose liners, while coolant systems need hoses rated for sustained high temperatures.
  • Pressure and temperature: Determine the maximum pressure and temperature your system will see. AN fittings are rated for pressures exceeding 1000 PSI in most sizes, but hose ratings vary widely. Always choose hose and fittings that exceed your system’s peak demands to build in a safety margin.
  • Routing: Map out the most direct path with gentle bends (see AN plumbing routing tips). Avoid sharp turns, kinks, and areas near exhaust manifolds, turbochargers, or other extreme heat sources. Maintain at least 1–2 inches of clearance from hot components. Allow extra length for engine movement under torque.
  • Component compatibility: Verify that every fitting, hose end, and adapter matches in both size and thread type. AN fittings use a 37° flare, but some combination fittings use a 45° flare (JIC). Also check for plumbing thread adapters (NPT, BSP, etc.) when connecting to OEM engine blocks or accessories.
  • Access for maintenance: Position fittings so that you can easily reach them with a wrench for future disassembly. Avoid placing fittings directly behind engine mounts or in tight corners where you can’t get a tool in.
  • Budget and availability: AN fittings can become expensive, especially if you use many specialty angles. Decide up front which connections truly need a swivel or bulkhead fitting and where a simpler straight adapter will work.

Selecting Hose and Fitting Sizes

AN sizes are denoted by a dash number that corresponds to the nominal inside diameter in sixteenths of an inch. For example, a -6 hose has a 6/16″ (3/8″) inner diameter, while a -10 hose is 10/16″ (5/8″). Choosing the correct size is a balance between flow requirements and packaging constraints.

Common AN Sizes for Engine Bay Systems

  • -4 (1/4″ ID): Oil pressure gauge lines, turbo oil feed lines, small fuel return lines, vacuum lines.
  • -6 (3/8″ ID): Most engine oil cooler lines, power steering pressure lines, small fuel supply lines (naturally aspirated setups).
  • -8 (1/2″ ID): Transmission cooler lines, large oil cooler lines, fuel supply for moderate power levels (up to ~600 hp).
  • -10 (5/8″ ID): High-flow fuel systems (boosted engines, high-horsepower builds), large coolant bypass lines.
  • -12 (3/4″ ID) and larger: Main coolant circulation, large oil return lines from turbos, or extreme flow requirements.

When in doubt, size up if space allows. Larger diameters reduce fluid velocity and pressure drop, but they also require larger fittings and hose bends. For most street and track builds, -6 and -8 sizes cover the majority of engine bay needs. Always consult the pump or component manufacturer’s recommended port size.

Types of AN Fittings

Understanding the different fitting types helps you build a system that routes cleanly and minimizes the number of joints (and potential leak points). Here are the most common ones you will encounter:

  • Straight adapter: A simple male-to-female or male-to-male connector for joining hose ends to component ports or extending lines.
  • 45° and 90° fittings: Angled adapters that allow you to change direction without a sharp hose bend. Use them to route around obstacles like intakes, alternators, or engine mounts. Avoid using more than two 90° bends in a single hose run to keep flow efficient.
  • Swivel fittings: A fitting that can rotate after tightening, allowing you to orient the hose at any angle before locking down the nut. Swivels are invaluable for hard-to-reach connections and reducing stress on the hose.
  • Bulkhead fittings: Used to pass a hose through a panel, firewall, or chassis rail. They have a threaded body with a nut on each side, providing a clean, sealed pass-through.
  • Union fittings: Join two male hose ends together, often used to extend a line or integrate a filter or sensor in the middle of a run.
  • Reducer / expander: Change from one AN size to another (e.g., -8 to -6). Keep transitions as short as possible to avoid flow restrictions.
  • Plug / cap: Seal unused ports on components like oil filter adapters, pumps, or fittings.

Also, pay attention to material. Most AN fittings are made from 6061-T6 aluminum (lightweight, corrosion-resistant) or stainless steel (stronger, more resistant to galling). Aluminum is sufficient for most automotive use and is easier to machine, but stainless steel is preferred for high-stress or high-heat areas (e.g., exhaust manifold connections). Never mix aluminum and stainless steel in threaded connections without anti-seize compound to prevent galling.

Hose Selection: Rubber vs. PTFE vs. Nylon

The hose itself is as important as the fittings. For engine bay use, the three main types are:

Rubber (Nitrile) Hose

The most common and affordable option. It has a synthetic rubber inner liner, a fabric braid for strength, and sometimes a stainless steel outer braid for abrasion resistance. Rubber hose is flexible, easy to cut, and compatible with most oils, coolants, and fuels. However, it is not suitable for extreme heat (above 300°F / 150°C) or for ethanol-rich fuels (E85) unless the liner is specifically rated. It also tends to shed particles over time. Use rubber hose for oil cooler lines, power steering, and general coolant bypass.

PTFE (Teflon) Hose

PTFE hose has a smooth inner lining that is chemically inert and can handle higher temperatures (up to 500°F / 260°C) and aggressive fluids like ethanol, methanol, or brake fluid. It is more rigid than rubber and requires special methods for cutting and assembly (a length of the hose is “crimped” to the fitting). PTFE is the best choice for fuel systems, especially with E85 or race gas. It also reduces weeping through the hose wall, which rubber can do over time. Note that PTFE hose is more expensive and requires a dedicated assembly tool (a hose cutter and a fitting press).

Nylon / Aeroquip AQP

Some high-end hose blends (like Aeroquip’s AQP) combine a rubber inner with a synthetic braid that offers excellent flexibility and high temperature resistance. These are often used in extreme racing environments but are overkill for most street cars. Check specifications carefully.

For 95% of custom engine bay builds, a high-quality stainless steel braided rubber hose (e.g., Earl’s Thermoguard or Russell PROCLASSIC) is a reliable choice. If you are building a dedicated race car or a flex-fuel vehicle, step up to PTFE.

Tools You Will Need

Installing AN fittings correctly requires a few specialized tools. Gather these before you start:

  • AN hose cutter: A dedicated cutter that slices through braided hose cleanly without fraying the wire. Circular saw blades or angle grinders can work but are messy.
  • Hose assembly fixture: A vice-mounted tool that holds the fitting while you thread the hose into it, preventing damage to the fitting threads.
  • Flare nut wrenches (crowfoot wrenches optional): AN fittings use hex nuts on the hose ends. A crescent wrench can round them off; use open-end wrenches of the correct size or a set of flare nut wrenches for tight spaces.
  • Torque wrench: AN fittings should be tightened to manufacturer-specified torque values. Overtightening can crack aluminum fittings; undertightening causes leaks.
  • Teflon tape or thread sealant (for NPT threads only): Do not use sealant on the AN flare itself—the metal-to-metal seal should be dry. Use a small amount of PTFE paste or tape on the male NPT threads only.
  • Deburring tool: For cleaning the inside and outside of cut hose ends to ensure a proper seal.
  • Safety glasses and gloves: Stainless steel braid can cause painful cuts.

Designing the Layout

With your components, fittings, and hose chosen, it is time to create a detailed routing diagram. Mark the exact positions of all ports on the engine, radiator, oil cooler, fuel pump, filter, and other components. Then draw the hose runs, noting where you can use straight, 45°, or 90° fittings. Keep these principles in mind:

  • Minimize bends: Every bend creates a restriction. Use the gentlest angle possible—prefer a 45° over a 90° if space allows.
  • Use bulkhead fittings for panel passes: Do not let a hose rub against a sharp sheet metal edge. Bulkheads protect the hose and provide a professional look.
  • Allow for movement: The engine and transmission will rock under load. Leave at least a few inches of slack in hoses that cross from the engine to the chassis, and use a flexible section at each end.
  • Cluster fittings for serviceability: Group multiple connections in a location where you can easily access all of them with wrenches. For example, mount all bulkhead fittings on a removable bracket.
  • Keep hoses away from exhaust: If a hose must cross near a manifold, wrap it with heat shielding (reflective sleeve) or use a metal heat shield.
  • Label everything: Use tags or tape to mark each hose with its function (e.g., “Oil Cooler Feed,” “Fuel Return”) during mock-up to avoid confusion during final assembly.

Installation Step-by-Step

Follow these steps for a leak-free installation:

  1. Pre-cut and dry-fit all hoses. Lay the hose alongside your diagram to confirm lengths. Leave 1–2 inches of extra length for adjustment. Use a sharp blade to cut the hose square.
  2. Deburr the cut ends. Remove any loose braided wire or rubber shards inside the hose that could break loose later and clog a small passage.
  3. Assemble the hose ends. Place the hose into the fixture, then thread the aluminum socket onto the hose by hand until it bottoms out. Then tighten the steel nut onto the cone—do not force it; it should go smoothly. Check that the flare is seated correctly.
  4. Install fittings onto components first. Use a wrench on the hex portion of the fitting, not the threaded flare. Tighten NPT adapters to the appropriate torque (usually 15–25 ft-lbs for -6 aluminum, more for larger sizes). Use thread sealant on NPT threads.
  5. Attach hose assemblies. Align the hose nut with the female flare on the component fitting and tighten by hand, then use a wrench to tighten a further 1/2 to 1 turn. Do not overtighten—just enough to compress the flare for a seal.
  6. Secure hoses with clamps or brackets. Use P-clamps with rubber inserts to hold hoses every 12–18 inches. Avoid metal-to-metal contact that can chafe the braid.
  7. Check clearance. Rotate the engine by hand (remove spark plugs) and watch for any hoses that contact pulleys, belts, or steering linkages. Adjust routing as needed.
  8. Pressure-test the system. Fill the system with the appropriate fluid (or air at low pressure) and check every fitting for leaks. Mark any seeping connections and tighten slightly. After a successful static test, start the engine and let it reach operating temperature while watching for leaks under pressure and heat.

Testing and Leak Prevention

A leak-free AN system comes down to careful assembly and retightening. After the initial test, check all fittings again after a few heat cycles—fittings can loosen slightly as temperature changes cause materials to expand and contract. Common leak causes include:

  • NPT threads not sealed properly (use correct sealant, not tape on flared connections).
  • Hose not fully seated into the end fitting (the hose shoulder must touch the bottom of the socket).
  • Flare nut not tight enough (it should bottom out against the shoulder, not be overtightened to the point of stripping).
  • Using the wrong fitting (e.g., a 37° AN flare on a 45° JIC component).

If you encounter a persistent leak, disassemble, inspect the flare for scratches, and reassemble with a new washer (if applicable) or new fitting. Never use silicone or RTV on AN connections—it will only mask the problem.

Maintenance Tips for Longevity

Once your system is installed and leak-free, it requires occasional attention:

  • Inspect hoses annually: Look for cracks, bulges, or frayed braid, especially near hot components or areas of flex.
  • Check torque on fittings after major events (track days, long trips). A vibration can loosen a fitting over time.
  • Use heat shielding where necessary: Reflective wrap works, but make sure it does not trap moisture against the hose.
  • Replace hose ends if they become damaged. A damaged flare or stripped nut will not seal properly.
  • Flush the system when changing fluids. Small debris can get trapped in fittings and cause restriction.

Final Thoughts

A custom AN fitting system is a hallmark of a professional engine bay. It improves reliability, simplifies maintenance, and adds significant visual appeal. By taking the time to plan your routing, select the right hose and fitting sizes, and follow correct assembly procedures, you can create a setup that performs flawlessly for years. For further reading on AN fitting standards and installation techniques, check out Russell Performance’s technical resources and AN Plumbing 101 guide. Whether you are building a weekend warrior or a podium contender, the details of your fluid system matter—invest the effort now, and your car will reward you with consistent performance and peace of mind.