AN bolts Decoded
AN hardware is the industry standard for aircraft bolts. AN means Army-Navy, the old designation for USA Military Specifications (MilSpec).
AN bolts are far different than common hardware bolts. The steel itself is far stronger than common bolts. They are case-hardened. Threads are not cut into AN bolts; threads are rolled in with a press, as if you were forming threads with your fingers in a rod of clay. Cadmium plating provides good corrosion resistance. They have industrial markings on the bolt heads. You should be aware, there are AN bolt counterfeiters. You can not tell a counterfeit bolt by looking. This problem was discovered when the USA had a rash of skyscraper window-cleaning scaffold failures, nationwide. Other high-stress applications had similar failures, all traced back to counterfeit AN bolts. Do not buy AN hardware by finding the best prices. Buy from reputable sources only.
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There is a .PDF file linked here, which you can download and print:
(NOTE: DO NOT use "fit to page" in the printer options, when you print out this chart. The "three inch ruler" on the left side of the chart MUST measure an accurate three inches or 76,2mm.)
This .PDF file will give you AN bolt sizes, by Part Number. The first digit after the AN letters in the Part Number will be the bolt diameter, in 1/16ths of an inch. The AN-3 bolts are 3/16” (~5mm) in diameter. AN-4 bolts are 4/16” or 1/4” (~6,4mm) in diameter. AN-5 bolts are 5/16” (~8mm) in diameter. You can measure the length of AN bolts in a glider with calipers, without removing the bolt from the airframe.
Bolt Sizing Calipers, using PDF Chart
If the next digit is the letter H, then the head has been drilled for safety wire. This is a great option if you ever need it, but HG manufacturers seldom (if ever) use bolts with H.
The next one or two digits will be the length of the bolt, from the bottom of the bolt head to the threaded end of the bolt. This is dimension Y on the .PDF bolt chart. If there is a single digit, it will be 7 or less, always. This will be the length of the bolt in 1/8ths of an inch. If there are two digits there, the first digit is the bolt length in whole inches. The second digit is 7 or less, always, indicating this number of one-eighth inches (which adds to the whole inch value). A 1/4” diameter bolt which is two and five eighths inches long will be called as AN4-25 (two inches, plus 5/8” in length). The AN5-37 bolt will be 5/16” in diameter, and three and 7/8” inches long.
The next digit of the Part Number, if any, will be A or nothing. AN bolt numbers ending in A do not have a hole in the threaded end, for a safety pin (the hole is Absent). AN bolt numbers without the A at the end will have a hole drilled in the threads, for the safety pin.
The threaded end of AN bolts will be tapered, to allow easier starting of the nut. Locking nuts must expose these tapered threads, above the nut, to be secure. Locking nuts can not grip the tapered threads sufficiently to be secure. The industry standard there is to have a minimum of two threads showing above the nut, to be “locked.” Nyloc nuts can not be re-used, once they are removed from a bolt, unless a safety ring or safety pin secures the nut. You can not restore the “locking” grip of any used Nyloc nut. All-metal locking nuts may be re-used.
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