Whenever an enthusiast changes anything in a car’s fuel system, fittings and hard-line sizing can become an issue. There are a couple of places where this can cause real confusion for DIYers. Fuel tanks, fuel pickups, and fuel-pump assemblies — along with fuel feed lines and fuel returns — are the most likely suspects. To read more about designing a complete fuel system, here’s an article that covers the details.
We talked with the experts at Tanks Inc., to get the lowdown on fuel fittings and hard-line requirements for fuel systems and components. From this discussion, we were given an awareness of the different types of threads and how they are used. While there are many different types of threaded fittings around the world, we are going to stay with the two most commonly seen in automotive systems: NPT and AN threads.
For obvious reasons, selecting the correct fuel fittings with the right threads is critical for making a leak-proof seal. A uniform threading system was developed in 1841 to solve incompatibility problems with various manufacturers. William Sellers defined the standard for nuts, bolts, and screws for American use in 1864. This eventually became the National Pipe Tapered Thread (NPT) that is still in use today.
What defines the NPT thread primarily is the 60-degree thread angle. Modern NPT threads have a tapered male and female thread which is sealed with Teflon tape, and in some cases, joint compound. Pipe thread sizes are based on the inside diameter of the fitting. This is the flow size and is most important to engineers working on flow dynamics.
An example of NPT thread sizing: If an NPT fitting lists the size as 1/2-14 NPT, it is identifying a pipe thread with a half-inch inside diameter and 14 threads per inch.
Keep in mind that NPT threads are tapered and typically are spec’d by the length of engagement. This means the fitting can only be tightened a certain distance before it jams. It is convenient to realize that the fitting probably will not tighten down to the point where no threads are showing.
A taper thread starts loose and tightens as the fitting is threaded in. Unlike a typical parallel or straight thread fitting, a taper thread will pull tight and make a fluid-tight seal when torqued as the flanks of the threads compress against each other. The common rule of thumb is to tighten down the fitting finger tight plus one or two turns with a wrench. Teflon tape is also recommended to ensure a leak-free seal.
Adapter fittings for NPT threads are prevalent and easy to find at most automotive or hardware stores making it easy to adapt from NPT to whatever type of line you choose. Tanks Inc., has adapters to go from NPT to AN here and NPT to hose barb here.
AN thread specification was derived from the US military as a standard agreed upon by the Army and Navy (AN). AN sizes were established by the Aerospace industry years ago and are designated by the outside diameter (O.D.) of the rigid metal tube that each size fitting is used with.
AN fittings are commonly used to connect flexible hose. AN sizing is designated by a dash (-) followed by a number. That number stands for the number of 1/16ths of an inch of the O.D. of a hard line. Since tubing and hoses can be found with various wall thicknesses, the designated size number does not necessarily tell you how large the inside diameter will be.
Each AN size number has its own standard thread size. These are the same thread sizes that have been used in aircraft and industrial applications for many years.
An example of AN thread sizing: a -6 AN line is 6/16-inch or 3/8-inch.
NPT, AN, And Hard-Line Size Compatibility
Unlike NPT fittings, hard-line sizes designate the O.D. of the line as its size. This is where the confusion comes into play for many enthusiasts. In the chart below we have outlined the nearest equivalent size between NPT, AN, and hard-line to help solve some of the confusion.
3/8″ hard line
1/2″ hard line
5/8″ hard line
If you understand all the thread designations and requirements, building a custom system is not a difficult task. If you still have questions and need some help, reach out to the folks at Tanks Inc. to get all the fuel system answers you need.