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An AN thread is relative to the size of the fitting. The AN designation refers to Army Navy since the fitting design was a joint effort between the two military branches. The AN size refers to the outside diameter of tubing that will properly correspond to the fitting. These thread sizes are easily corresponded with Society of Automotive Engineers (SAE) thread sizes. The AN fittings use a special 37-degree flare tested and found to be superior to the common SAE flared fittings, which are a 45-degree flare.
While an AN thread fitting refers to the outside diameter of the tubing used with the fitting, a National Pipe Threads (NPT) fitting refers to the inside diameter of the fitting. AN thread fittings can be connected to SAE threads and vice-versa; however, NPT threads must use an adapter to connect with AN thread fittings. The reason for this is that the NPT threads are tapered and are narrower at one end than the other. This allows the fittings to be connected in a flare-type method when tightened. The SAE and AN thread fittings are straight-cut threads with no taper.
The AN thread fittings have been widely used by the military, the U.S. National Aeronautics and Space Administration, and the airline industry as well as the racing community. Most high-horsepower engines use the AN fittings exclusively for the plumbing of vital fluid lines such as oil, fuel and coolant. The success of these fittings has been undisputed in nearly every form of racing competition worldwide. It is critical that only special aluminum wrenches be used on the fittings to prevent damage. By using a steel wrench on the aluminum fittings, the corners are easily rounded off and the brightly anodized red and blue finish is destroyed.
Unlike many other fasteners, when assembling the AN thread fittings, a small drop of light oil must be applied to the threads to prevent galling and damage as well as the sticking and freezing of the two halves of the fitting. The leading cause of failure to the AN fitting is over-tightening. The 37-degree flare of the fitting does not require extreme force to make a secure seal. Just making the fittings snug, then torquing a slight bit will be sufficient to create a leak-proof seal in most cases. One of the most critical steps to take when assembling the AN thread fittings is to ensure the threads and flare of the fitting are clean and free of contaminants during assembly.