AN Fittings – The Complete Guide
From what "AN" actually stands for, to the exact torque spec for your -AN8 union – everything you need to build leak-free plumbing on your car or truck.
What is AN?
AN stands for Army-Navy. The standard was developed jointly by the United States Army and Navy during World War II to create a unified, interchangeable system for hydraulic and fuel line fittings across military aircraft and ground vehicles. The goal was simple: any fitting from any manufacturer would connect perfectly to any hose or port using the same standard.
The standard is formally defined as MIL-DTL-5070 and related military/aerospace specifications. After the war, it was adopted by motorsport, hot rodding, and eventually the wider performance automotive world. Today you'll find AN fittings on everything from Formula 1 cars to weekend track cars to underground fuel tanks on a diesel race truck.
The defining characteristic of AN fittings is the 37° flare seal – a precisely-machined conical surface on both the male and female halves that form a metal-to-metal seal when tightened. This completely eliminates the need for O-rings, gaskets, or thread sealant in the joint itself, making connections extremely reliable, leak-resistant, and re-usable hundreds of times.
AN fittings (37° flare) are different from JIC fittings (also 37° flare but a different thread pitch in some sizes) and SAE 45° flare fittings, which are not interchangeable. Always verify the flare angle when adapting between systems.
The Dash Size System
AN fittings are identified by a "dash" number (written with a minus sign: -3, -4, -6, -8, -10, -12, -16, -20, -24). The dash number corresponds to the hose/tube outside diameter in 16ths of an inch.
-8
Dash number
8/16" = 1/2"
Tube O.D. (imperial)
Simple rule: dash number ÷ 16 = tube outside diameter in inches
The most commonly used sizes in automotive performance applications are:
- -4Small fuel feed & return, sensor lines, oil pressure
- -6Main fuel feed (up to ~400 hp), oil cooler, power steering
- -8Fuel feed (400–800 hp), oil catch-can, coolant bypass
- -10Fuel (800 hp+), oil drain & return, large coolant lines
- -12Fuel cell vent, large oil drains, remote reservoirs
- -16Dry sump tanks, very large return lines, bulk storage
When in doubt about which size to use, size up rather than down. A -8 line that is "too big" will flow freely; a -6 line that is "too small" will starve your fuel pump or restrict oil flow under high load.
The 37° Flare – How AN Seals
Every AN male fitting has a 37° conical nose precision-machined to exact tolerances. The female AN swivel nut contains a matching 37° concave seat. When the nut is tightened, the male nose presses into the female seat and the two metal surfaces make intimate contact across the full circumference – forming a seal without any soft material (gasket, o-ring, or tape) in the joint.
This is why AN fittings can be assembled, disassembled, and re-assembled hundreds of times without leaking – as long as both flare surfaces remain undamaged.
37° AN Flare
Used on all true AN fittings. Metal-to-metal seal. No sealant needed.
45° SAE Flare
Common on residential plumbing and some OEM lines. NOT interchangeable with AN.
NPT Taper Thread
Seals on thread interference. Requires sealant. Used on ports/blocks – adapted to AN via fittings.
Materials & Finish
AN fittings are available in several materials. Choosing the wrong one for an application is one of the most common (and potentially dangerous) mistakes builders make.
6061 Aluminium (Anodised)
Weight
????? Lightest
Corrosion Resistance
????? Good
Cost
$ Budget-friendly
Max Temp
150°C
Use for
Oil, fuel, coolant, air – general performance builds. Avoid strong acids/alkalis.
Avoid on
Brake fluid (DOT 3/4/5.1), power steering fluid (degrades anodising over time)
Stainless Steel (316L)
Weight
????? Heavier
Corrosion Resistance
????? Excellent
Cost
$$$ Premium
Max Temp
315°C
Use for
Brake lines, turbo oil returns, exhaust manifold sensors, marine/salt environments
Avoid on
Nothing – stainless is compatible with virtually all fluids
Titanium
Weight
????? Very light
Corrosion Resistance
????? Excellent
Cost
$$$$$ Race-grade
Max Temp
300°C
Use for
Weight-critical race applications only. Minimal strength advantage over aluminium at higher costs.
Avoid on
Overkill for street use
Carbon Steel (Black)
Weight
????? Medium
Corrosion Resistance
????? Poor – rusts
Cost
$ Cheapest
Max Temp
200°C
Use for
Dry air, hydraulic oil only. Not for fuel or coolant systems on performance builds.
Avoid on
Any wet fluid system – rust contamination risk
AN Fitting Types
The AN system covers a wide family of fitting shapes. These are the most commonly used in automotive builds:
Male Union (Bulkhead Coupler)
AN-815Connects two female AN hose ends inline. Usually hex-body so you can hold it while tightening both ends.
Female-Female Swivel Union
AN-818Two female AN openings. Used when both lines terminate in male fittings.
Tee Fitting (Three-Way)
AN-819Splits or merges flow at one point. Common on fuel rails and manifolds.
Straight Hose End
0° endConnects directly onto the hose at 0° departure. Use where you have a straight run with room.
45° Hose End
45° endA 45° departure from the hose axis. Great for crowded engine bays.
90° Hose End
90° endSharp 90° turn at the end. Use for ports where a straight hose would conflict with adjacent components.
Adjustable Swivel Hose End
Banjo/SwivelCan be rotated to any angle and locked with a jam nut. Extremely useful for alignment-critical ports.
NPT-to-AN Adapter
NPT-ANBridges an NPT-threaded port (engine block, filter head, manifold) to the AN system. Uses thread sealant on the NPT side only.
Bulkhead Fitting
BulkheadPasses a line through a firewall or panel. Uses a lock nut on the non-sealing side to clamp against the panel.
O-Ring Boss (ORB) Adapter
ORBThreads into an ORB port (most modern aluminium blocks/heads) and converts to AN. Seals on the face O-ring, not the thread.
Hose Compatibility
Not all hose is compatible with all fluid types. Match the hose to the service it will see:
| Hose Type | Inner Liner | Max Pressure | Fluid Compatibility | Best Use |
|---|---|---|---|---|
| Stainless Braided (PTFE liner) | PTFE/Teflon | 2,500 psi (-6) | All fluids including brake fluid, methanol, exotic fuels | Brakes, fuel, turbo oil, hydraulics |
| Stainless Braided (rubber liner) | Nitrile / EPDM | 1,000 psi | Fuel, oil, coolant, air. NOT brake fluid. | General purpose oil, fuel, coolant |
| Nylon Braided (lightweight) | Nitrile / CPE | 350–500 psi | Fuel, oil, coolant, air. NOT brake fluid. | Non-critical lines where weight matters |
| Push-On (no crimp/screw) | Nitrile/CPE | 250 psi | Fuel, oil, coolant, air – low pressure only | Return lines, vents, catch-can connections |
Brake lines are safety-critical. Always use PTFE-lined stainless braided hose with stainless steel AN end fittings for brake applications. Aluminium fittings degrade with glycol-based brake fluids and must not be used.
NPT to AN Adapters
Most engine blocks, oil pans, fuel rails, and filter housings use NPT (National Pipe Thread) ports – a tapered thread that seals through thread interference (not a flare). To connect your AN plumbing to these ports, you need an NPT-to-AN straight adapter.
The NPT side uses PTFE tape or anaerobic thread sealant (e.g. Loctite 567). The AN male side then interfaces with your hose end in the normal metal-to-metal 37° flare manner – no sealant.
| NPT Size | Equivalent AN | Typical Applications |
|---|---|---|
| 1/16" NPT | -3 | Sensor ports, gauge fittings |
| 1/8" NPT | -4 | Common sensor/sender port size |
| 1/4" NPT | -6 | Fuel pressure regulator, block ports |
| 3/8" NPT | -8 | Fuel pump inlets, oil cooler ports |
| 1/2" NPT | -10 | Return lines, larger block ports |
| 3/4" NPT | -12 | Remote oil filter, dry sump tanks |
| 1" NPT | -16 | Large reservoir ports, bulk fittings |
NPT is a tapered thread – install it with 2–3 wraps of PTFE tape on the male thread, or one drop of anaerobic thread sealant. Thread in by hand, then 1–2 turns with a wrench. Do not over-tighten NPT fittings into aluminium ports – you will crack the housing.
Step-by-Step Installation
Follow these steps in order for a clean, leak-free build every time.
Cut the hose cleanly
Use a proper hose cutting tool or fine-tooth hacksaw with a sharp blade. The cut must be perfectly square – never use scissors or a knife, which compress the liner. For spiral-wound PTFE hose, use a rotary cutter to prevent fraying. Deburr aluminium hose ends after cutting to prevent liner damage.
Inspect the flare seat on both fittings
Examine the 37° flare seat inside the female AN swivel nut and on the male fitting body. Any nicks, burrs, or corrosion will cause a leak regardless of torque. Light scratches can be polished out with fine grit (1000+). Deep gouges mean replace the fitting.
Assemble the hose end onto the hose (reusable fittings)
For reusable screw-type ends: lightly lubricate the socket threads with clean assembly oil or the fluid the line will carry. Thread the socket counterclockwise (left-hand thread) onto the hose braid – count the turns, typically 3–4 for -AN4 and up. Insert the nipple into the hose and thread into the socket clockwise by hand until snug, then use two wrenches – hold the socket, turn the nipple – until correct depth is reached (nipple hex should be flush with or 1–2mm proud of the socket hex).
Route and measure before final assembly
Dry-fit the entire line before making anything permanent. Ensure adequate radius on bends (minimum 5× the hose I.D. for stainless braided, 3× for push-on). Keep lines away from heat sources (exhaust, headers) or protect with heat sleeve. Avoid contact with sharp chassis edges – use grommets or split plastic conduit.
Connect and torque the AN union
Thread the male AN fitting into the female port by hand first – if it resists, stop and check for cross-threading. Never force. Once hand-tight, use two AN wrenches (not adjustable spanners): hold the body fitting, turn the swivel nut. Tighten to "finger-tight + 1/6 turn" as a starting point, then verify against the torque table. Do NOT over-torque – you will deform the 37° flare and still get a leak.
Pressure test before running the engine
For fuel and oil lines: pressurize the system to operating pressure with a hand pump or by briefly running the engine at idle. Inspect every joint with a cloth or UV dye. For brake lines: bench-bleed with fluid, apply and hold pedal pressure for 30 seconds and check for any weeping at the unions. Fix any leaks before driving.
Torque & Size Reference
Use this table for all AN fitting dimensions and torque specs. Values are given for aluminium fittings; stainless steel can be torqued approximately 10–15% higher due to the harder sealing surface.
| Dash | Tube O.D. | Thread | Bore | Wrench (AN) | Wrench (Hose) | Torque |
|---|---|---|---|---|---|---|
| -3 | 3/8" | 3/8"-24 UNF | 4.8mm | 1/2" | 7/16" | 8–9 Nm |
| -4 | 7/16" | 7/16"-20 UNF | 6.3mm | 9/16" | 23/32" | 11–13 Nm |
| -5 | 1/2" | 1/2"-20 UNF | 7.9mm | – | – | 14–15 Nm |
| -6 | 9/16" | 9/16"-18 UNF | 9.5mm | 11/16" | 7/8" | 15–17 Nm |
| -8 | 3/4" | 3/4"-16 UNF | 12.7mm | 7/8" | 1-1/32" | 28–31 Nm |
| -10 | 7/8" | 7/8"-14 UNF | 15.9mm | 1-1/32" | 1-7/32" | 34–37 Nm |
| -12 | 1-1/16" | 1-1/16"-12 UNF | 19mm | 1-1/4" | 1-7/16" | 44–47 Nm |
| -16 | 1-5/16" | 1-5/16"-12 UNF | 25.4mm | 1-1/2" | 1-3/4" | 68–73 Nm |
| -20 | 1-5/8" | 1-5/8"-12 UNF | 31.8mm | – | 2-5/16" | 83–88 Nm |
| -24 | 1-7/8" | 1-7/8"-12 UNF | 38.1mm | – | – | 95–100 Nm |
Bookmark this page – or print the torque table and keep a laminated copy in your tool chest. Getting the torque right is the single biggest factor in leak-free AN systems.
Common Mistakes & How to Avoid Them
Using PTFE tape on AN threads
Why it's a problem
AN fittings seal on the 37° metal flare, not the threads. PTFE tape compresses the flare geometry and causes leaks. Never use thread sealant on AN male/female unions.
The fix
No sealant needed. Clean, correct torque, and undamaged flare seats are all that is required.
Mixing aluminium and stainless in fuel systems
Why it's a problem
Not a problem in itself, but different torque specs and thermal expansion rates mean connections may loosen over time in high heat-cycle environments like turbo oil drains.
The fix
Match materials within each sub-system where possible. If mixing, re-check torque after the first 5 heat cycles.
Over-torquing with adjustable spanners
Why it's a problem
Adjustable spanners bite into hex flats at points, not evenly. This rounds off the softer aluminium hex and makes subsequent removal very difficult or impossible.
The fix
Use proper AN crowfoot or open-end wrenches (1/16" increments). Invest in a decent AN wrench set – it pays for itself on the first job.
Bending stainless braided hose too tightly
Why it's a problem
Bending below minimum radius kinks the inliner, restricts flow, and creates stress fractures in the braid over time.
The fix
Reposition end fittings (use a swivel or different angle end) so the hose takes a gentle arc rather than a tight bend.
Not supporting long hose runs
Why it's a problem
Unsupported braided hose vibrates against chassis members and wears through quickly, especially on track cars.
The fix
Clamp at regular intervals (every 30–40cm), use protective sleeving where contact is unavoidable.
Using aluminium fittings on braided brake lines
Why it's a problem
DOT 3/4/5.1 brake fluids are glycol-based and aggressively corrode anodised aluminium over time, leading to fitting failure – potentially while braking.
The fix
Always use stainless steel fittings and PTFE-lined hose for brake systems. No exceptions.
Ready to build?
Browse our full range of AN fittings, hose ends, adapters, and braided lines. Every item ships with genuine spec sheets.