"AN" Thread Sizes (Army/Navy Threads)
Commonly used on Nitrous Oxide systens. The sizing chart and numbering system was deviloped during World War II where the armed forces used Nitrous Oxide systens to boost the power output of aircraft engines.
AN sizes, originally developed for use by the U.S. Armed forces ("A" for army and "N" for navy), describe the outside diameter (O.D.) of tubing in 1/16-inch increments. For example, an AN 2 fitting will fit a tube with an O.D. of 2/16", or 1/8", while an AN 8 fitting will fit a tube with an O.D. of 8/16", or 1/2". Because the actual thickness of tube walls can vary from brand to brand, the inside diameter of a tube is not used as a reference. You will also find the dash (-) symbol or the word "dash" itself used in conjunction with AN sizes. A "dash six" fitting translates to AN-6.
Each AN fitting has an established thread sizing. The following chart shows the relationship between AN size, tube O.D., and SAE thread size:
"NPT" Thread Sizes (National Pipe Taper)
NPT sizes are the most commonly used fitting sizes for general plumbing, piping, and tubing use; not quite as popular as AN for automotive use, but still very common. While AN fittings depend on the outside diameter of a tube for sizing, NPT fittings depend on the interior diameter (I.D.) of the fitting itself. The following chart shows the each size's thread-per-inch count, the I.D. of the fitting, and the AN fitting size with the closest-matching I.D.(inside dimension).
Inch (SAE) to Metric Conversion
Download All these charts in one HTML page for easier displaying.
Click on this link - AN_NPT_Metric_SAE.zip