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 controlling document

AN, MS, NAS, AS and SAE are the government and industry specification families FAA AC 20-62E gives as examples of standard parts: parts made "in complete compliance with an established U.S. Government or industry-accepted specification, which includes design, manufacturing, and uniform identification requirements" [1]. Many AN numbers were superseded by MS numbers (AN470 by MS20470, AN426 by MS20426) [2], and many MS standards were then converted to NASM with the part number kept identical: MS90353U's cancellation notice says "the part numbers have not been changed" [3]. Not every MS has an NASM successor, so the decoder only lists NASM where a conversion is documented.

The dash numbers encode size, and the units vary

On a solid rivet, MS20470AD3-5 means material AD (2117-T), 3/32" diameter and 5/16" length (AC 43.13-1B para 4-57a(3)) [2]. On blind fasteners the last dash is usually grip, and the unit depends on the maker: maximum grip in 16ths for CherryMAX [4] and Hi-Lok [5], thousandths for Composi-Lok [6], and nominal grip in 1/16" increments for Visu-Lok II (Table 3 gives the range) [7]. Confirm the unit against the controlling document before the PO goes out; the decoder shows the source for every scheme it reads.

OEM specs

Airframers also maintain their own part standards. If your drawing calls out an OEM standard, that is the part you buy, even when a NAS or MS part looks equivalent.

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.