AN Fitting vs NPT Fitting: What's the Difference?

Quick Answer

AN fittings use a straight thread with a 37-degree flare that seals metal-to-metal, while NPT fittings use tapered threads that seal by deforming and need sealant. Choose AN for high-pressure fuel lines you will take apart, NPT for permanent ports in pumps, regulators, and blocks, and use an AN to NPT adapter when the two must meet.

If you are plumbing a fuel system, the two thread standards you will meet most often are AN and NPT, and mixing them up causes leaks that are hard to chase. They are not interchangeable, and the difference is not just the thread pitch.

This comparison covers how each one seals, where each belongs, and how an AN to NPT adapter lets you connect them without guessing. It is written for anyone building or repairing a fuel system who wants the connection to hold the first time.

How the Two Sealing Methods Differ

AN fittings seal at a 37-degree flare. The threads are straight and only hold the fitting together; the seal happens when the male flare seats into the female cone. Because the seal is mechanical and repeatable, AN fittings can be disassembled and reassembled many times without replacing the fitting.

NPT fittings seal on the threads themselves. The threads are tapered, so as you tighten them the threads wedge together and deform slightly to close the gap. That is why NPT joints need thread sealant and why they are not meant to be taken apart repeatedly.

• If a joint will be serviced often, AN is the better choice.
• If a port is permanent and rarely opened, NPT is usually fine.

Where Each Standard Belongs

AN fittings dominate aftermarket fuel systems because they handle high pressure, resist vibration, and come in a predictable size range. You will see them on fuel rails, regulator inlets and outlets, and hose ends. Their straight threads and flare seat make them reliable in applications where a leak is a safety issue.

NPT shows up on pumps, blocks, gauges, and some factory-style ports because it is inexpensive and easy to tap. It is a good match for a fixed connection that does not need to come apart. The trade-off is that once an NPT joint is tightened and sealed, reusing it can be risky.

• Check the port before buying: a tapered hole points to NPT, a machined cone points to AN.
• Do not force an NPT fitting into an AN port or the reverse.

Adapters: Making AN and NPT Work Together

You do not have to choose one standard for the whole car. An AN to NPT adapter joins a flare connection to a threaded port, which is common when an aftermarket AN line meets a pump or regulator with an NPT inlet. The adapter does the translation so each side keeps its own sealing method.

When you use an adapter, seal only the NPT side with the appropriate sealant and leave the AN side clean. Putting sealant on the flare can prevent the metal-to-metal seat from sealing and can push debris into the fuel system.

• Match the AN size and the NPT size exactly; they are separate dimensions.
• Keep the flare side dry and torque it to the fitting maker's spec.

Sizing and Thread Identification

AN sizes are based on sixteenths of an inch, so a 4AN fitting corresponds to a 1/4-inch nominal size and an 8AN to 1/2 inch. NPT sizes are nominal pipe sizes and do not match the actual thread diameter, which is why a 1/16 NPT port is much smaller than it sounds.

When you are unsure, measure the thread outside diameter and compare it to a known fitting. Guessing between similar sizes is the most common cause of a fitting that threads in a turn or two and then stops.

• A fitting that only threads partway by hand is the wrong size, not tight.
• Keep a known-good AN and NPT fitting on the bench for comparison.

Trade-Offs at a Glance

• Sealing method: AN seals at a 37-degree flare with straight threads; NPT seals on tapered threads and needs sealant. AN is more repeatable; NPT is simpler for a one-time connection.
• Serviceability: AN fittings can be taken apart and reused; NPT joints can degrade each time they are loosened. Choose AN anywhere you expect to open the line again.
• Cost and availability: NPT ports are common and cheap to produce; AN fittings and adapters cost more but cover more sizes and angles. Budget for adapters if you are mixing standards.
• Leak risk: AN leaks usually trace to a damaged flare or wrong size; NPT leaks often come from too little or too much sealant. Inspect the flare and use sealant only on the NPT side.

Fittings That Show the Difference

• 4AN to 1/16" NPT: Connects a 4AN male flare to a 1/16" NPT male thread; anodized stainless steel construction; listed under AN Fitting and tagged in Vacuum Reference Fittings. A clear example of an AN to NPT adapter: the flare side seals metal-to-metal while the NPT side threads into a port. Useful for vacuum reference and small fluid connections where this size combination is required.
• 8AN to 1/2" Ford EFI Quick Connect: Joins an 8AN male flare to a 1/2" Ford male EFI quick connect; anodized matte black finish; category EFI Quick Connects and Hardline Adapters. Shows that AN hardware is not limited to threaded ports. It adapts an AN flare line to a factory-style quick connect, so you can run AN plumbing without cutting the factory connection.
• 10AN Aluminum Hard Tubing 20ft Anodized Clear: 10AN aluminum hard tubing, 20 ft length, clear anodized finish, listed under AN Fitting in Hose & Hose End Fittings. Hard line is a natural fit for AN flare connections because the flare can be formed on the tube end. The 20 ft length and clear anodized finish suit long runs where you want a durable, corrosion-resistant line.

Which Should You Use?

Best for

• High-pressure fuel lines that will be serviced or modified
• Builds using AN hose ends, rails, and regulators
• Connecting AN lines to factory quick connects with an adapter

Not for

• Permanent ports where an NPT tap already exists and will not be opened
• Situations where you cannot verify the thread size before assembly
• Anyone expecting an AN and NPT fitting of similar number to interchange

AN and NPT solve the same problem in different ways, and the right choice depends on whether the joint needs to come apart. Match the sealing method to the port, use an adapter when the two must meet, and verify the size before you tighten anything.