Decoding AN, MS, NAS, NASM & OEM part numbers

A fastener part number is a compressed spec. Learn to read the prefix and the dash codes and most of the catalog stops being a wall of cryptic strings.

The prefix tells you the standard family

AN (Army-Navy) is the WWII-era legacy family, still seen in general aviation. MS (Military Standard) tightened tolerances and materials. NAS (National Aerospace Standard) covers the high-strength, precision fasteners most structural work uses. NASM is the converted-military version of many MS specs. Each prefix points to a controlling document that fixes material, finish, strength, and tolerance.

The dash numbers encode size

After the base number, dash codes give diameter and grip. A common convention: diameter in 1/32" (so a -6 is 6/32" = 3/16") and grip length in 1/16". But conventions vary by standard and product line, so always confirm against the controlling spec rather than assuming. The decoder treats size as approximate for exactly this reason.

OEM specs supersede

Airframers maintain their own families — Boeing BAC/BACR, Airbus ABS/ASNA/EN/NSA — that often supersede or augment the NAS/MS world for a given program, with full traceability to a Qualified Products or Manufacturers List. If your drawing calls an OEM spec, that wins, even when a commodity NAS equivalent exists.

Why it matters for selection

The part number is where the material, head style, and strength you selected get pinned down — and where sourcing starts, because distributors stock by part number. Paste one into the decoder to jump straight to the family.