Plumbing 101: Part 1

1-2 — Instead of rubber hose, go straight to high performance AN plumbing. Once you get the hang of working with the various hoses and fittings you’ll never turn back.

In the distant past, this was “state-of-the-art” when it came to plumbing. Today, we highly recommend you stay away from rubber hose and worm clamps.

Decades ago, the task of plumbing a car usually meant you went to the local auto parts store, bought a roll of fuel safe rubber hose along with a handful of common brass fittings. You hooked it all together with “fits-all” worm gear clamps. An hour or two later you had a “race car” fuel system done. It wasn’t exactly safe; it wasn’t exactly pretty and it wasn’t exactly efficient. But then hot rodders and racers discovered aircraft plumbing. It proved very safe, it proved very good-looking and it turned out to be extremely efficient. Our world forever changed (and it’s still evolving, but we’ll get to that later).

Instead of rubber hose, go straight to high performance AN plumbing. Once you get the hang of working with the various hoses and fittings you’ll never turn back.

Before we go any further, let’s start from the very beginning. Aircraft (AN) hose really began the in mid 1930’s. At that time, a universal military standard was developed to more easily identify and categorize hardware, tubing, hose and hose (end) fittings. This standard became A-N or “Army-Navy” (some figure it was “Air Force-Navy” or “Air Corps-Navy, but it doesn’t matter). The A-N standard provides us a universally accepted method of identification and standard specifications for aircraft fittings, hose and even hardware.

The hard line in the photo is ½-inch. Meanwhile the AN hose is -8AN. They’re not quite the same size – AN is slightly smaller on the ID than hardline. Keep that in mind when selecting hose for something like a fuel system.

Following WW2 early rodders and racers discovered military surplus – it was big (and cheap) after the war. The truth is, the use of aircraft hose and fittings was a very good practice in the late forties, and it’s still a good practice today. Within the A-N system, the various components are identified by way of a specification number and a series of letters and dashes identifying the size of the component. When it comes to hose and fittings, the following chart depicts the relationship between A-N numbers, metal tube outside diameters (OD’s) and thread size:

Plenty of aftermarket parts are sized with pipe threads. Pipe threads incorporate a tapered joint in order to seal. You’ll always encounter pipe thread connections on automotive parts. There’s no getting around it.

AN Number, Tube Size & Corresponding Thread Size

A-N SIZE METAL TUBE SIZE THREAD SIZE

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2 1/8-inch 5/16-24 SAE

3 3/16-inch 3/8-24 SAE

4 1/4-inch 7/16-20 SAE

5 5/16-inch 1/2-20 SAE

6 3/8-inch 9/16-18 SAE

8 1/2-inch 3/4-16 SAE

10 5/8-inch 7/8-14 SAE

12 3/4-inch 1-1/16-12 SAE

16 1-inch 1-5/16-12 SAE

20 1-1/4-inch 1-5/8-12 SAE

24 1-1/2-inch 1-7/8-12 SAE

28 1-3/4-inch 2-1/4-12 SAE

32 2-inch 2-1/2-12 SAE

NOTE: You’ll note A-N sizes 2 and 5 on the above chart. Fittings and hose in these sizes are next to impossible to find or are prohibitively expensive. Because of this it’s not really a viable hose/fitting size for automotive applications.

Pipe thread joints leak if they aren’t sealed. As a result, you have to use Teflon tape or Teflon paste to make the seal. Earl’s sells both tape and paste.

So far so good but the threads are straight. What’s up with that? Aircraft or AN fittings seal by way of a cone shape machined into the respective ends of the fittings. One end is “male” while the other is “female”. Adjoined AN ports do not require any form of sealant in order to function without leaks. On the other hand, almost all automotive and motorcycle applications use pipe threads. Because of this, parts with pipe threads aren’t compatible with the A-N fittings and adapters must be used. Rather than confusing the issue, companies such as Earl’s Performance, and others never size fittings by the thread size. Instead, they simply name the fitting size by the dash number (A-N number or size). Because of this, a No. 8 fitting is simply that — a dash 8 (or “number” 8) fitting. Adapters, required to fit pipe thread parts to AN parts are named for their application. For example, to simplify the transition from NPT (National Pipe Thread) to A-N “dash” sizes, something like a straight adapter fitting with 3/8-pipe on one end and a -8 thread on the other, is termed “3/8-inch NPT to No. 8”. If you have difficulty translating hose size into fractions, try this: Place the A-N size over sixteen and reduce the number (or divide). Here’s an example:

#8 hose = 1/2-inch or 0.5-inch

When plumbing a hot rod or race car, you’ll always keep in mind the relationship between pipe threads and dash numbers: A large percentage of components (OEM and aftermarket) don’t have A-N ports. Instead, they’re based on tapered pipe thread. The truth is pipe thread is the bane of our hobby. Here’s why: “National Tapered Pipe” thread sizes refer to the inside diameter of a piece of pipe that receives the male end. It’s designed for an interference fit between the male and female threads (hence the taper in the two parts). Theoretically, this interference fit within the taper is designed to stop leaks. The trouble is, the theory doesn’t work. When joining any pipe thread, you must use Teflon tape or paste on the threads to prevent leaks. And FYI, when it comes to choosing between Teflon tape and Teflon paste, the paste is often easier to use (but it’s definitely messier). The following chart shows the various sizes of pipe threads along with a comparable (closest) A-N fitting size:

Pipe Thread Sizes Versus AN Fitting Sizes

Pipe Threads Theoretical Closest

Thread Size Per Inch I.D. of Fitting A-N Fitting To Size

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1/16″ 27 1/16″ N/A

1/8″ 27 1/8″ #4

1/4″ 18 1/4″ #6

3/8″ 18 3/8″ #8

1/2″ 14 1/2″ #10

3/4″ 14 3/4″ #12

1″ 11-1/2 1″ #16

1-1/4″ 11-1/2 1-1/4″ #20