How military standard hardware numbering actually works — base numbers, dash numbers, letter codes, NSN cross-references, and the supersession traps that stall purchase orders.
If you buy aircraft or military hardware for long enough, you will meet the same physical item under several names: an AN number on a 1960s parts catalog page, an MS number on the current standard, an NSN on the government supply record, and sometimes a NAS or AS number after the standard changed hands. None of these are arbitrary. Each family has a logic, and once you can read it, a part number stops being a string to copy-paste and becomes a description of the part — often precise enough to catch an ordering error before the hardware ships. This guide covers the major families, how to decode a base number plus dash number, why the same item carries both a part number and a National Stock Number, and the pitfalls that generate most of the wrong-part returns we hear about from A&P mechanics and MRO purchasing desks.
The oldest system still in daily use, created during World War II so the services could buy common hardware to one drawing. AN numbers cover bolts, nuts and washers, fittings, and general airframe hardware. AN bolts (AN3–AN20) and AN flared-tube fittings remain active and are stocked everywhere general aviation hardware is sold. Many other AN designs were superseded by MS numbers decades ago — more on that below.
The workhorse family. An MS number points at a Military Standard sheet: a one-or-few-page drawing that completely defines a part design plus a table of variants. Most of the screws, nuts, washers, rivets, and O-rings on a military or transport-category aircraft carry MS numbers. Note that many MS sheets have since been transferred out of DoD custody: some became SAE AS documents and others became NASM documents (for example, MS24665 cotter pins are now maintained as NASM24665) — usually keeping the same digits, which is a mercy.
An industry standard series published by the Aerospace Industries Association rather than the government. NAS hardware tends toward the high-strength and close-tolerance end: NAS bolts in 160–180 ksi tensile ranges, precision screws like the NAS1352 socket-head series, and structural panel fasteners. NAS also absorbed some superseded AN designs — the everyday AN960 flat washer's successor is NAS1149.
Standards published by SAE International. Some AS numbers are original SAE designs; many others are former MS sheets adopted by SAE when DoD divested standards it no longer wanted to maintain, typically keeping the legacy number (MS becomes AS, digits unchanged). If a distributor tells you an MS number is "canceled," check whether it was actually converted to an AS or NASM document — the part is usually alive and well under the new prefix.
Numbers like M83248/1-010 point at a slash sheet under a military specification — here, slash sheet 1 of MIL-DTL-83248 (fluorocarbon O-rings), dash 010 selecting the AS568 size. The pattern is M[spec number]/[sheet]-[dash]. You will see these constantly on seals, packings, electrical wire, and connectors. The slash sheet defines the configuration; the parent spec defines the material and qualification requirements.
A-A numbers (for example A-A-59126) are Commercial Item Descriptions: the government's way of buying ordinary commercial products — commercial hardware, tapes, cleaners — without writing a full military specification. A-A documents describe salient characteristics rather than a complete design, so two compliant A-A items from different makers may differ in details an MS part never would.
Every family follows the same grammar. The base number identifies the standard sheet that fixes the design. The dash number (and any letter codes) selects one line from the variant table on that sheet — a size, a length, a material, a finish. The critical rule: the dash code's meaning is defined per standard. A dash 4 on one base number and a dash 4 on another may have nothing in common.
MS20219-4 decodes as base MS20219 — the Military Standard sheet for a single-groove phenolic control-cable pulley with an integral ball bearing — plus dash 4, which selects the specific size variant from the sheet's table. Every MS20219-x part is the same design at a different size. The government supply system carries this exact item as NSN 3020-00-277-1119; the part-number page and the NSN record describe one and the same pulley.
AN bolts encode dimensions directly in the number. The base digit gives the diameter in sixteenths of an inch: AN3 is a 3/16-inch (10-32 thread) bolt, AN4 is 1/4-inch, and so on up to AN20. The dash number encodes grip/length per the AN standard's table — AN3-5 is the 5 length increment of the 3/16 bolt. The trailing A means the shank is undrilled (no cotter-pin hole); omit the A and you get a drilled shank. Order AN3-5 when you meant AN3-5A and you will receive a bolt with a hole in it.
Here the variant coding is a letter plus digits with no dash at all. Base MS20995 is the safety (lock) wire standard; C selects corrosion-resistant steel as the material; 32 is the nominal diameter in thousandths — 0.032 inch, the most common lockwire size in aviation. MS20995C41 is the same wire at 0.041 inch. Same grammar, different syntax: you cannot parse it without the sheet, but with the sheet it is unambiguous.
When you have a number and want the decode plus a published price, our part number lookup resolves MS, NAS, AN, and M-prefix numbers to their stock records.
A part number describes a design. A National Stock Number identifies an item of supply in the federal cataloging system. When the government decides to stock a standard part, DLA assigns a 13-digit NSN: the first four digits are the Federal Supply Class (5305 for screws, 5310 for nuts and washers, 5331 for O-rings), the next two are the country code of the cataloging bureau, and the final seven are a serial item identifier. The NSN exists so every service, depot, and contractor orders exactly the same item the same way, regardless of which standard's part number appears on their drawing.
For sourcing, the cross-reference runs both directions and is genuinely useful. If a distributor no-quotes the part number, searching by NSN often surfaces stock listed only under the government identifier — and vice versa. A superseded part number (say, AN365) and its successor (MS20365) frequently share a single NSN, which is the cleanest proof available that the two numbers are the same item of supply. Our catalog is browsable both ways: by stock number and by part number, at the same published price either way. For the practical mechanics of using cross-references to widen a search, see our companion guide on sourcing aircraft hardware for owners and MRO shops.
Type-certificated aircraft fly with parts catalogs written fifty years ago, so superseded numbers never die. The classics: AN365 self-locking nuts became MS20365 (and AN364 thin nuts became MS20364); AN380 cotter pins became MS24665 (now NASM24665); AN960 flat washers were superseded by NAS1149, with a size recoding that trips people — AN960-10 corresponds to NAS1149F0363P, not to any NAS1149 number containing a 10. Keep a supersession cross-reference at the purchasing desk, and when in doubt let the shared NSN arbitrate.
One trailing letter can change drilled versus undrilled (AN bolts), material class, or thread form. On many screw and nut standards a C means corrosion-resistant steel while the unlettered part is cadmium-plated alloy steel — different strength, different corrosion behavior, different price. Quoting "MS35206 screws" without the full dash number is quoting a family, not a part.
Cadmium, zinc-nickel, phosphate, passivated, black oxide — encoded differently on every standard, sometimes as a letter, sometimes folded into the dash table. Two parts differing only in finish code are different items of supply with different NSNs. If your drawing predates cadmium restrictions, verify whether the current standard revision substituted the plating.
AN bolt dash numbers encode length; AN fitting dash numbers encode tube size in sixteenths; MS20219 dash numbers select a pulley size; M83248/1 dash numbers are AS568 O-ring sizes. Same syntax, four unrelated meanings. Decode against the governing sheet, every time.
Finally, know when to leave the standards system entirely. If the item you need is a modified standard part, an obsolete design with no active source, or a print-defined component, that is a machining job, not a hardware buy. Our U.S.-based CNC machining services and aviation machining capability make parts to your drawing — a separate, domestically produced line of work from the stock hardware catalog. Send the print for a quote.
All three are standard part families. AN (Air Force-Navy) is the oldest, dating to World War II; MS (Military Standard) largely superseded it for military hardware; NAS (National Aerospace Standard) is an Aerospace Industries Association industry standard, typically covering higher-strength or more specialized aerospace hardware. A given design usually lives in one family, but many AN designs were later reissued as MS or NAS numbers.
The base number (for example MS20219) identifies the standard sheet that fixes the design. The dash number selects one variant from that sheet's table — a size, length, material, or finish. The meaning is defined per standard: on AN bolts the dash encodes length, on MS20219 it selects the pulley size. Never assume a dash code means the same thing across base numbers.
The part number identifies the design per its standard; the NSN identifies the item of supply in the federal cataloging system. When the government stocks a standard part, DLA assigns a 13-digit NSN so every service orders the same item the same way. The item can be ordered by either identifier — our part number lookup and NSN catalog resolve to the same records.
Some are active, some superseded. AN bolts (AN3–AN20) and AN fittings remain widely used. Others were superseded decades ago: AN365 nuts became MS20365, AN380 cotter pins became MS24665, and AN960 washers were superseded by NAS1149. Drawings still call out the old numbers, so cross-referencing is routine.
Letter codes are defined per standard and typically select material, finish, or a feature option. On AN bolts, a trailing A means an undrilled shank. On MS20995 safety wire, C means corrosion-resistant steel. Always decode letters against the specific standard sheet, not by analogy to another family.
A practical guide for owners and MRO shops.
MS, NAS, AN, and M-prefix numbers with published pricing.
Browse stock numbers by Federal Supply Class.
MS and NAS machine screws in stock.
Self-locking nuts, flat washers, lock washers.
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