AN Fitting Size Guide

If you've ever ordered AN fittings and ended up with the wrong size, you're not alone. AN sizing trips up even experienced builders — because the dash number refers to the outside diameter of the hose, not the inside, and it has nothing to do with thread pitch. Get it wrong and you're looking at leaks, flow restriction, or fittings that simply won't connect.

This guide covers everything you need to size AN fittings correctly for your build — whether you're plumbing a fuel system, oil cooler, or brake lines.

What Does "AN" Mean?

AN stands for Army-Navy — a joint US military standard adopted post-WWII and later embraced by motorsport for its reliability and standardisation. Every AN fitting uses a 37° JIC flare seat, which is what creates the leak-free metal-to-metal seal.

The dash number (e.g. -6AN, -8AN) represents the hose outside diameter in 1/16 inch increments. So -6AN = 6/16" = 3/8" OD hose. Simple once you know the rule.

AN Dash Size Chart

Use this table to match your hose or port to the correct dash size:

Most common for performance builds: -6AN (fuel), -8AN (oil), -10AN (oil cooler/coolant).

AN6 swivel fitting in blue/red anodised — a common size for fuel feed lines.

AN8 to -10 ORB straight adapter — used to connect AN lines to O-ring boss ports on engine blocks and pumps.

An adjustable AN wrench — essential for tightening fittings without marking the anodising.

How to Measure Your Existing Fittings

If you're replacing an existing fitting and aren't sure of the size, measure with digital calipers:

1. Measure the outside diameter of the male thread (not the female port)
2. Match to the table below:

Common mistake: measuring the female port opening instead of the male thread OD — this gives a smaller reading and leads to ordering the wrong size.

AN vs NPT vs BSP — Don't Mix Them Up

AN fittings use a 37° JIC flare. NPT (National Pipe Taper) and BSP (British Standard Pipe) use tapered or parallel threads with a sealant-dependent seal. They can look similar and threads may partially engage — but mixing them causes leaks and can damage the seating surface.

• AN/JIC: 37° flare seat, no sealant needed, reusable
• NPT: tapered thread, requires PTFE tape or sealant, common on US engine blocks
• BSP: parallel or tapered thread, common on European and Japanese engines

If you're connecting AN lines to an engine port, you'll likely need a thread adapter. Always identify the port thread standard before ordering. For a full breakdown, see our AN Fitting Thread Guide: JIC, NPT, BSP and ORB Explained.

A thread adapter like this M10x1 to AN3 S/S unit lets you connect AN lines to metric engine ports cleanly and without sealant.

Choosing the Right AN Size for Your Application

Flow rate is the key variable. Undersizing restricts flow under load; oversizing adds unnecessary weight and cost. Here's a practical guide by system:

Fuel Lines

• Naturally aspirated up to ~300bhp: -6AN
• Forced induction or high-flow injectors: -8AN feed, -6AN return
• High-power builds (500bhp+): -8AN or -10AN throughout

Oil Lines (cooler, filter relocation)

• Standard: -8AN
• High-volume engines or long runs: -10AN

Coolant Lines

• Heater core bypass, catch tank: -10AN
• Radiator hoses: -16AN to -20AN

Brake Lines

• Always -3AN — do not upsize brake lines

AN3 S/S bleed nipples — the correct size for all brake line applications.

Straight, 45°, or 90° — Which Angle Do You Need?

The angle of your hose end fitting determines how cleanly you can route hose without tight bends. Tight bends restrict flow and stress the hose — use angled fittings at fixed connection points instead.

• Straight: use where the hose runs directly away from the port with no bend needed
• 45°: use where a gentle change of direction is needed — common at oil pans and filter housings
• 90°: use in tight spaces where the hose needs to turn immediately — engine bays, bulkhead exits
• 120°/180°: use for very tight routing or where a U-turn is needed without a hose bend

A 90° AN8 hard line fitting — ideal for tight engine bay routing without stressing the hose.

120° swivel fittings give you even more routing flexibility — the swivel feature lets you align the hose after assembly.

Common AN Sizing Mistakes

• Buying before measuring: always measure the port or existing fitting first
• Mixing AN and NPT: threads may partially engage but the seal will fail — always identify thread standard before ordering
• Undersizing for power level: -6AN on a 600bhp fuel system will restrict flow under hard acceleration
• Confusing hose ID and OD: AN dash size refers to hose OD — don't measure the inside bore
• Ignoring swivel fittings: fixed hose ends can make assembly difficult in tight spaces — swivel ends let you align after tightening

Ready to Order?

Now you know your dash size, thread standard, and the angle you need — you have everything required to order with confidence. Browse the full Maxflo Motorsport range below, available in aluminium and stainless steel from -3AN to -20AN:

→ Shop Maxflo Motorsport

Also in this series:

• PTFE vs Stainless Steel Braided Hose — Which Should You Choose?
• AN Fitting Thread Guide: JIC, NPT, BSP and ORB Explained
• How to Size an Oil Cooler for Your Build