AN fittings explained for first-time vehicle performance builds
If you are building or upgrading a performance vehicle, AN fittings are likely to appear somewhere in your fuel, oil or cooling system. They give you a secure way to connect hoses and components while making it easier to route lines neatly through a crowded engine bay. For a first-time build, the main challenge is understanding the sizing system, connection types and hose compatibility before you start ordering parts.
What AN Fittings Are Used For
AN fittings are threaded connectors commonly used with fuel lines, oil coolers, catch cans, transmission coolers and other fluid systems. Most use a 37-degree flare, which creates the sealing surface between two matching components rather than relying on the threads themselves to prevent leaks.
When you start comparing parts, you will often see AN fittings sold alongside compatible hose ends, adapters and hoses. Looking through options like Aeroflow automotive performance products and accessories can help you see how these components are matched by size and connection type before you begin assembling your own setup. The important part is making sure every fitting suits the hose, fluid and component it connects to.
How AN Fitting Sizes Work
AN sizes are shown as dash numbers, with common examples including -4AN, -6AN, -8AN and -10AN. The number originally relates to tube outside diameter in sixteenths of an inch. For example, -6AN corresponds to 6/16 inch, or 3/8 inch.
That does not mean every -6AN fitting has exactly the same internal diameter. Internal passages vary between fittings and hose designs, so you should choose a size based on the flow requirements of the system rather than assuming a larger fitting is always better.
How to Match AN Fittings With Hose
The fitting needs to suit the construction of the hose you are using. A hose end intended for braided rubber hose may not work with PTFE hose, which uses a different inner liner and normally requires fittings designed specifically for that material.
You should also confirm that the hose size matches the AN fitting size. Using parts from compatible systems reduces the risk of leaks, damaged hose liners or weak connections. This is particularly important in fuel and oil systems where a failed connection can quickly become a serious mechanical or safety issue.
Which Fitting Angles Make Sense
AN hose ends are available in straight and angled configurations, with 45-degree, 90-degree and 180-degree fittings commonly used in performance builds. Angled fittings can help you route hoses around engine components without forcing the hose into tight bends.
The simplest route is usually the best one. Too many bends can make installation more complicated and may introduce unnecessary restrictions. Before buying fittings, plan the hose path and check where each connection sits in relation to heat sources, belts, pulleys and moving components.
How to Install AN Fittings Properly
Before assembly, inspect the fitting threads and flare seats for damage. Start threaded connections by hand so you can feel whether the parts are aligned correctly. Cross-threading or forcing a fitting into place can damage the sealing surfaces and make leaks more likely.
AN connections are a type of flare fitting, where the angled surfaces create the seal through metal-to-metal contact rather than the threads themselves. You should therefore avoid automatically applying thread sealant to the AN connection. Some adapters also include NPT threads, which seal differently and may require an appropriate sealant, making it important to identify each connection type before assembly.
What to Check Before Final Assembly
Before tightening everything permanently, check that each hose has enough length to accommodate engine movement and that none of the lines are rubbing against sharp edges or hot components. You should also confirm that fittings are properly seated and that hoses are not twisted under tension.
Once the system is assembled, inspect every connection during the first pressure test. Finding a small leak before the vehicle is driven is far easier than dealing with one after heat, vibration and pressure have been introduced.
Getting AN Fittings Right From the Start
AN fittings become much easier to work with once you understand how size, hose type, thread style and routing fit together. If you plan the system before buying parts and check compatibility at every connection, you are far more likely to end up with reliable fuel, oil or cooling lines that are easier to install, inspect and maintain.
