AN hex head bolts are commonly used in aviation applications are manufactured to the "Army-Navy" (AN) military standard, predecessor to "Mil-Spec" (MS) and "National Aerospace Standard" (NAS) which followed. They are available in steel and stainless steel versions with drilled or undrilled shank.
AN steel bolts are made of high-strength type 4037 or 8740 alloy steel, they are centerless ground, threaded after heat treatment and cadmium plated per specification QQ-P-416A, Type II, Class 3 for superior corrosion resistance as compared to commercial grade zinc plated bolts.
AN stainless steel bolts are made of 431 Stainless Steel (CRES) specified by letter "C".
For bolts with drilled head add letter "H" after the AN number, (eg. AN4H-15).
Minimum Tensile Strength (Steel and CRES) - 125,000psi.
AN bolts are offered with shank drilled for cotter pin or undrilled for stop nut application, and with or without drilled head for safety wire. AN bolts with undrilled shank are specified by letter "A" after the dash number. The length of AN aircraft bolts is measured from under the head to the end of the shank. The "grip" is the unthreaded portion of the shank. Threads are pressed into the steel, not cut, thus maintaining strength throughout the entire shank. The surface under the head of AN bolts is designed to be a bearing surface to reduce non-uniform stress as the bolt is torqued thus reducing premature fatigue.
AN3 through AN20 (REV. 12), Military Specification Sheet: Bolt -Machine, Aircraft [superseded by NASM3 through 20]. This specification covers aircraft machine bolts part numbers, AN3, AN4, AN5, AN6, AN7, AN8, AN9, AN10, AN14, AN16, AN17, AN18, and AN20. AN3 THRU AN20 is canceled and superseded by NASM3 THRU NASM20. The Part Identification Number (PIN) for AN3 THRU AN20 REV. 12 and the part number for NASM3 THRU NASM20 are identical. The part numbers have not been changed.