In the automotive plumbing world, there a few different ways to measure thread size on fittings and hoses. The most popular thread measurements are AN and NPT. What’s the difference between the two? How does an AN thread convert to a “plain English” fractional value? How do the two measurements relate to one another?
Those are some of the more common questions we hear, and we’ll attempt to answer them with an assist from the folks at Earl’s Performance.
Tapered vs. Non-Tapered Thread
Aside from thread sizes, the most important difference between AN and NPT threaded fasteners is that NPT uses a tapered thread. That’s a fancy way of saying the bolt is actually slightly smaller at the tip than the base. It’s designed that way to firmly seat the male and female threaded ends together.
With a tapered NPT thread, you must use an additional thread sealant to ensure a leak-free connection. As a general rule, plastic (PVC) fittings should get PTFE tape wrapped on the threads. Metal threads should be treated with a sealant paste (aka pipe dope).
On the other hand, AN fittings are flare-type compression fittings and, as such, don’t require any form of thread sealant.
Make sense?
This ultra-realistic rendering of tapered vs. non-tapered thread we just made in MS Paint might clear things up.
“AN” Thread Sizes
“AN” (Army-Navy) sizing was established many years ago by the U.S. military to set a common standard for designating hose size. This standard of measurement is based on the nominal outside diameter (O.D.) of the rigid metal tube that each size fitting is used with. The nominal outside diameter of the tubing is expressed in sixteenths of an inch increments and then given an equivalent dash number. For example, an -8 AN fitting translates to an 8/16 inches, or 1/2-inch.
You can convert any AN fitting to its nominal inch size by dividing the AN number by 16 as follows: -4 AN = 4/16″ or 1/4″.
Since tubing and hoses have assorted wall thicknesses, depending on manufacturers, the designated size number does not necessarily tell us anything about the inside diameter (I.D.) of the plumbing component. Also, each AN size number has its own standard thread size, which is depicted in the chart below:
AN Fitting/Tubing Specs
| AN Size | Tube O.D. Nominal Equivalent | Thread Size (SAE) | Typical Applications |
|---|---|---|---|
| -2 | 1/8″ | 5/16″-24 | |
| -3 | 3/16″ | 3/8″-24 | |
| -4 | 1/4″ | 7/16″-20 | Small fuel lines, gauges, brakes, compressed air, vacuum lines |
| -6 | 3/8″ | 9/16″-18 | Fuel lines, transmission, hydraulic power, return lines, compressed air |
| -8 | 1/2″ | 3/4″-16 | Oil lines, fuel lines, transmission lines, hydraulic power return lines |
| -10 | 5/8″ | 7/8″-14 | Oil Lines, fuel lines, transmission lines, heater hose, power steering lines |
| -12 | 3/4″ | 1-1/16″-12 | Oil lines, fuel lines, transmission lines, power steering lines, heater hoses |
| -16 | 1″ | 1-5/16″-12 | Oil lines, power steering lines, heater hose |
| -20 | 1-1/4″ | 1-5/8″-12 | Oil lines, power steering lines, coolant |
| -24 | 1-1/2″ | 1-7/8″-12 | Oil lines, coolant |
| -28 | 1-3/4″ | 2-1/4″-12 | Oil lines, coolant |
| -32 | 2″ | 2-1/2″-12 | Coolant |
“NPT” Thread Sizes
National Pipe Threads (NPT) are the next most popular thread size used in competition plumbing, according to Earl’s. Unlike the AN sizes, NPT numbers actually correspond with the inside diameter (I.D.) of the fitting. Review the chart below :
NPT Fitting/Tubing Specs
| Pipe Thread Size | Threads Per Inch | Theoretical Fitting I.D. | Closest AN Fitting |
|---|---|---|---|
| 1/16″ | 27 | 1/16″ | |
| 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 |
| 1-1/2″ | 11-1/2 | 1-1/2″ | -24 |
| 2″ | 11-1/2 | 2″ | -32 |
The fact that NPT threads are tapered would be worth a lot to the beginner like me who learned the hard way. Cracked the trans case tightening the cooler lines. AN fittings fan for life!!
Good call Bob. We added a new section on tapered threads and included a basic diagram to illustrate the differences too.
Is there such a thing as a metric size fitting? I assume it’s not an AN. Thanks
Yes there is and you’re correct–it’s not AN size. You’ll typically see metric fasteners/fittings described as something like “M16 x 1.5,” in which “M16” refers to the diameter of the fastener and the “1.5” refers to the thread pitch.
Here’s a handy reference page from FittingsInc.com that includes a Metric Thread chart as well as a handy guide to identify metric fittings.
Yes there is. BPP. British parallel pipe and one other one for hydraulic systems.
It’s actually BSPP: British Standard Parallel Pipe, and it’s definitely not metric, as it is measured in fractions of an inch.
There is also BSPT: British Standard Tapered Pipe. Which is very similar to NPT, but not interchangeable.
[…] NPT fittings are common in compressed air systems because they feature tapered thread. If you don’t know what that means, read this AN vs. NPT thread story. […]
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I have seen some people use NPT male threads in a NPTS threads. Such as the cooler line threads in the case of a 200R4 or 700R4 Transmission. These are not NPT they are NPTS (straight). Some brake line check valves are NPTS (female) threads, people try and use NPT fittings to tie into their brake lines and they leak.
Great job explaining this issue. Most custom fabs such as race cars are all using AN fittings, so everyone must learn to become familiar with them and to start using them in their projects. I am facing just such an issue with my ‘antique’ Camaro having cooling issues. Thanks so much for this informative article.
[…] Quick Tech: “AN” vs. “NPT” Fittings and How They Relate […]
An does not relate to threaded fasteners that falls under the SAE specs
hence the terms “relative” and “theoretical”
A great explanation of NPT tapered sizing. Trying to directly fit AN to NPT or SAE is a dubious adventure. They don’t match up very well. You may be able to force it to fit. but it will surely leak. Trying to fit inverted flare and compression fittings, to AN tubing, is useless. Unless Cross threading is your goal.
thx helped alot
Very helpful thank you so much
Very helpful, concise, detailed information. Much appreciated! Information like this is what the internet used to be great for. Now searching for just about anything ends with curated junk.
Again many thanks!
Your explanation of “an” vs “npt” is the only of many that makes real sense to anyone especially us that dont or I’ll say after reading your article “didn’t” know same with your sizing guide thanks gear heads your awesome
“You can convert any AN fitting to its nominal inch size by dividing the AN number by 16 as follows: -4 AN = 4/16″ or 1/4″. ”
This conflicts with the last chart conveying the NPT to AN closest inner diameter match.