AN/MS Aircraft Hardware Decoder

A bolt from a parts list: AN4H12A. AN4 is 4/16 = 1/4" with a 1/4-28 fine thread. H means the head is drilled for safety wire. 12 is 1 inch and 2 eighths, so the bolt is 1-1/4" long under the head. The thread on an AN4 is 7/16" long, which leaves 1-1/4 − 7/16 = 13/16" of grip: the thickness it is meant to clamp. The final A means an undrilled shank, so it takes an AN365-428A self-locking nut torqued to 50–70 in-lb.

Choosing a rivet for two 0.040" sheets. Three times the thickest sheet is 0.120", and the next standard size up is 1/8" (a -4). The rivet must be long enough for the grip plus 1.5 diameters to form the shop head: 0.080 + 1.5 × 0.125 = 0.268". Rounded up to sixteenths that is 5/16" (-5), so the rivet is MS20470AD4-5, drilled with a #30 drill, at least 1/4" from the edge and 3/8" apart.

The common mistake: reading the dash number as the grip. An AN6-12 is 1-1/4" long, but its 3/8-24 thread takes 9/16" of that, so the grip is only 11/16". Ordering by length for a 1-1/4" stack would leave the threads bearing in the hole. Pick by grip, then add up to 1/8" of washers to take up the difference.

AN standards were written jointly by the US Army and Navy so that both services could buy the same aircraft parts from any supplier. After the Air Force became a separate service in 1947 the letters were read as Air Force–Navy, which is how the FAA’s older mechanics’ handbook explains AN3DD5A. The numbers were designed to be read on the hangar floor: diameter in sixteenths, length in eighths, material and drilling as letters.

Military Standard (MS) numbers later replaced many AN parts, which is why MS20470 supersedes AN470 and MS20365 sits beside AN365. NAS numbers are National Aerospace Standards written by the aircraft industry itself. When the Defense Department handed many of its standards to industry in the 1990s, some MS parts took a NASM prefix, which is why the FAA’s AC 43.13-1B lists blind rivets as NASM20600.

The rivets changed shape over the same years. The universal head was developed for aircraft to replace round and brazier heads, and the 100° countersunk head became the US standard for flush riveting. Alloy letters were stamped into the heads so a mechanic can tell a soft AD rivet (a dimple) from a hard DD “icebox” rivet (two dashes) that must be kept refrigerated until it is driven.

This decoder reads AN3–AN20 hex bolts, AN and MS solid rivets, AN310–AN365 and MS21042, MS21044 and MS21045 nuts, AN960, AN970 and NAS1149 washers, AN500-series screws, AN380 and MS24665 cotter pins, MS20995 safety wire and AN815–AN823 tube fittings. Other families such as clevis bolts, eyebolts and internal-wrenching bolts are named and described, because their dash numbers are lookups on the standard sheet rather than a rule.

The coding rules come from the FAA’s AC 43.13-1B (chapters 4 and 7), AC 65-9A and the Aviation Maintenance Technician Handbook (FAA-H-8083-31B). Grip lengths, washer sizes and cotter-pin tables come from Aircraft Spruce’s published catalogue. This is a reading aid, not approved data: the aircraft’s maintenance manual, parts catalogue and structural repair manual always come first. Everything runs in your browser.

Related tools: Weight & Balance Calculator, Bolt Torque & Preload Calculator and the Socket & Wrench Size Converter.