How to Choose the Right AN Fitting Size for Your Fuel System

Quick answer

Size AN fittings by matching the hose dash number to the flow your fuel system needs, then confirm the flare and thread type at every connection. For most street and mild track builds, 6AN feed and 6AN return lines cover the job; step up to 8AN or larger when your pump, injectors, or horsepower target demands more volume. Always measure the hose end and the mating port before ordering.

AN fittings are sized in dash numbers, and that number is the first thing to get right. A 6AN hose is not the same as a 6AN port in every case, and a mismatch between hose size and flare size is the fastest way to leak fuel or restrict flow.

This guide walks through the practical decisions: how to read an AN hose size chart, when to step up a size, how flare and thread compatibility work, and where a few real DeatschWerks parts fit into a typical fuel system build.

Start With the Dash Number, Not the Thread

AN sizes use a dash number that refers to the nominal hose inside diameter in sixteenths of an inch. A 6AN hose is roughly 3/8 inch inside diameter, an 8AN is roughly 1/2 inch, and a 10AN is roughly 5/8 inch. The dash number tells you flow capacity; the thread size tells you what it screws onto. They are related but not interchangeable, so check both before you buy.

A simple AN hose size chart helps here: match the dash number to the hose you already have or plan to run, then confirm the fitting's flare and thread match the port. If you are replacing a factory line, measure the outside diameter of the hard line and the port thread rather than guessing from the engine bay.

• Write down your target flow and line length before choosing a dash size.
• Measure the mating port thread, not just the hose end, when replacing factory lines.
• Keep feed and return sizes consistent with your pump and regulator ports.

Match Fitting Size to Fuel Demand

Smaller AN sizes keep fuel velocity up and reduce the chance of aeration, but they can become a restriction at high flow. As a rule of thumb, 6AN feed lines support many mild street builds, 8AN feed lines are common on higher-power forced-induction setups, and 10AN or larger feed lines appear on dedicated race cars with large pumps. Return lines can often be one size smaller than the feed, but check your regulator's return port first.

The trade-off is real: going too large adds weight, cost, and fittings that are harder to route, while going too small can starve injectors at wide-open throttle. If you are unsure, size to the pump outlet and the fuel rail inlet, then work backward through the regulator and filter.

• Feed line size should match or slightly exceed the pump outlet size.
• Return line size should match the regulator return port, not the feed line.
• When in doubt, choose the size that matches your existing rail and pump ports.

Flare, Thread, and Compatibility Checks

AN fittings use a 37-degree flare, which is different from the 45-degree flare found on some plumbing and industrial fittings. They will not seal correctly if mixed. Threads are usually UNF, but the dash number and thread size must both match: a 6AN fitting has a different thread than an 8AN fitting, and adapters are often needed to bridge the two.

Compatibility also covers materials and hose type. PTFE hose needs PTFE-specific hose ends with a ferrule or olive, while rubber hose uses a different style. If you are building a line from scratch, buy hose ends and hose from the same size family and confirm the hose end includes the ferrule you need.

• Never mix 37-degree AN flares with 45-degree flares.
• Match the hose end style to the hose material: PTFE versus rubber.
• Check that adapters are the correct dash size on both ends.

Where Real Parts Fit In

A typical fuel system uses several AN connections: pump outlet, filter inlet and outlet, rail inlet, regulator inlet and return, and sometimes a gauge or service valve. Each connection is a chance to get the size wrong, so lay out the whole path before ordering. The DeatschWerks 30-Deg. 8AN Swivel PTFE Hose End is an example of an 8AN hose end with a female flare swivel and a 30-degree bend, which helps route PTFE hose away from tight spots without adding an extra adapter.

Filters and injectors also have their own sizing and flow implications. The 5 micron, 160mm, In-line fuel filter element fits a DW 160mm housing and is a universal stainless element, so it does not change your AN size but it does affect how much flow reaches the rail. Injector size, such as a matched set of 650cc injectors for a Nissan Skyline RB20DET, sets the fuel demand your feed line must support.

• Sketch the full fuel path before buying any AN fittings.
• Use a swivel hose end when routing tight bends to reduce adapter count.
• Confirm filter element size and housing fitment before finalizing line size.

Decision rules for AN fitting selection

• Match dash number to hose ID: Use the dash number as your starting point: 6AN is about 3/8 inch, 8AN is about 1/2 inch, 10AN is about 5/8 inch. Confirm the hose end and the port use the same dash size before ordering. If the hose and port dash numbers differ, you need an adapter, not a different hose end.
• Size feed and return separately: Feed line size should support your pump and injector demand; return line size should match the regulator return port. A common pattern is a feed line one size larger than the return, but verify against your actual components. Do not assume feed and return are the same size.
• Confirm flare and thread compatibility: AN fittings use a 37-degree flare and UNF threads. Check both the flare angle and the thread size, because a 6AN and 8AN fitting have different threads even though both are AN. A fitting that threads on but has the wrong flare will leak.
• Choose hose end style by hose material: PTFE hose needs a PTFE-compatible hose end with a ferrule or olive, while rubber hose uses a different style. Buying the wrong style means the assembly will not seal. Match the hose end to the hose, then match the dash size to the port.
• Lay out the full path before buying: List every connection: pump, filter, rail, regulator, gauge, and service valve. Count the fittings and adapters you actually need so you do not order a size that only fits one end of the system. A complete parts list prevents a second order and a stalled install.

Product facts that affect AN sizing

• 30-Deg. 8AN Swivel PTFE Hose End: 8AN hose end with a female flare swivel connection, a 30-degree bend, and an included ferrule/olive in an anodized matte black finish. The 8AN dash size sets the hose and port match, the 30-degree bend helps route PTFE hose in tight engine bays, and the included ferrule means you do not need a separate olive for assembly.
• 5 micron, 160mm, In-line fuel filter element: Stainless steel 5 micron in-line fuel filter element that fits DW 160mm housing and is universal. It confirms the filter end of your fuel system without changing AN size, but the 5 micron rating and 160mm length must match your housing before you finalize line routing.
• 650cc - Skyline RB20DET: Matched set of 6 650cc injectors for Nissan Skyline RB20DET 89-94. Injector flow capacity defines the fuel volume your feed line must deliver, so a 650cc matched set is a useful reference point when deciding between 6AN and 8AN feed sizes.

Who this AN sizing approach fits

Best for

• Street and mild track builds replacing factory fuel lines with AN hose and fittings
• Enthusiasts upgrading a fuel pump, filter, rail, or regulator and needing to match sizes
• Anyone building a PTFE or rubber AN fuel line from scratch and wanting a clear sizing rule

Not for

• Dedicated race cars with custom dry-sump or methanol systems that require engineering-level flow calculations
• Owners who only need a direct factory replacement line and do not plan to change hose size
• Builds where the pump, rail, and regulator ports are already fixed and cannot accept AN adapters

Pick the dash number that matches your hose and your flow target, then verify the flare, thread, and hose end style at every connection. A short parts list and a tape measure will save more time than any guess. If your build grows later, AN fittings make it easier to step up a size without replacing the whole system.