AN, MS and NAS part numbers explained
The short answer
An aircraft hardware part number usually names a standard, not a manufacturer. NAS6203-22 does not mean "part 6203-22 made by NAS" — it means "the part described in National Aerospace Standard 6203, in the -22 configuration". Any manufacturer approved to build to that standard can supply it, and two parts from two different manufacturers carrying the same number are meant to be interchangeable.
That single fact explains most of the confusion in aerospace procurement: you are buying a specification, and the manufacturer is a separate question.
What each prefix means
AN — Air Force–Navy Aeronautical Standard. The oldest family still in daily use, dating from the 1940s joint Army/Navy standardisation effort. You see it on basic hardware: AN3 through AN20 bolts, AN960 washers, AN310 castle nuts. Many AN standards have been formally superseded by MS or NAS equivalents, but the AN numbers persist in maintenance manuals and in everyday shop language, so they remain very much orderable.
MS — Military Standard. Issued by the US Department of Defense. Broader in scope than AN, and covers everything from fasteners (MS21042 self-locking nuts) to O-rings (MS28778) to electrical hardware. A large number of MS documents were later transferred to industry custodianship — which is where the next prefix comes from.
NAS — National Aerospace Standard. Published by the Aerospace Industries Association, not by the military. NAS standards tend to cover higher-strength or more closely toleranced parts than their AN equivalents — NAS6203, for instance, is a close-tolerance bolt series.
NASM — the merger. When the DoD moved a Military Standard into industry custodianship, the AIA re-issued it with an NASM prefix. NASM21287 is the industry-custodied version of what was MS21287. Both numbers commonly appear against the same part, and treating them as unrelated is a frequent source of duplicate line items on a bill of materials.
BAC / BACB — Boeing. An OEM's own standard rather than an industry one. BACB30NN is a Boeing bolt series. Airbus, Embraer and the engine OEMs all maintain equivalent private families. These are not interchangeable with the public standards unless the OEM says so.
What the dash number does
The number after the dash is the configuration, and it is not a free-text suffix. In most fastener families the dash number encodes diameter and length — commonly the first digit or two give the shank diameter in sixteenths of an inch, and the rest gives grip length. In an O-ring family such as MS28778, the dash number is a size code that maps to a specific inside diameter and cross-section.
The encoding is specific to the standard, which is the trap. There is no universal rule that converts a dash number into millimetres. -6 on a bolt standard and -6 on an O-ring standard describe entirely different things. Always read the dash number against the standard it belongs to, not against a general rule of thumb.
Suffix letters carry meaning too — commonly material, finish or a drilled-head variant — and again the meaning is defined per standard.
Why this matters when you order
A standard number alone may not be a complete order. Two parts can share a standard number and differ in material, plating or lockwire provision. If the maintenance manual calls out a suffix, it is load-bearing information, not an optional detail.
Superseded does not mean unavailable. A number can be inactive for new design while remaining in production for spares. If your manual calls out an AN number, that is what your aircraft is approved for, whether or not a newer NAS equivalent exists.
Cross-references are not always symmetrical. MS21287 and NASM21287 are the same part. AN960 and its MS equivalent may differ in a detail that matters for your application. Treat a cross-reference as a starting point for verification, not as an answer.
The certificate is separate from the number. A part number tells you what the part should be. An authorized release certificate — an EASA Form 1 or an FAA Form 8130-3 — tells you that this specific batch was released by an approved organisation. You need both, and the second is what an auditor asks for. We wrote separately about the difference between EASA Form 1 and FAA 8130-3 and about how to verify one.
Frequently asked
Is a NAS part better than an AN part?
Not better — different. NAS standards often specify closer tolerances or higher strength than the AN part they replaced, but "better" only means anything relative to what your maintenance manual approves. Fit the part the manual calls out.
Can I substitute MS21287 with NASM21287?
These are the same standard under two custodians, so in general yes. Confirm against your maintenance documentation before substituting anything, and keep the paperwork showing which you received.
Who actually manufactures an NAS part?
Any manufacturer qualified to build to that standard. This is why the same number reaches you from different sources at different prices and lead times, and why the release paperwork matters more than the brand on the bag.
How do I find a part if I only have a partial number?
Send us what you have. A partial number plus the aircraft type and the assembly it came from is usually enough for our sourcing team to identify the part and come back with options.
Sourcing AN, MS, NAS, NASM or OEM-standard hardware? Angad Aviation sources aircraft hardware to published standards from certified manufacturers, with traceability and, where applicable, an authorized release certificate you can verify online. Browse aviation hardware — bolts, nuts, washers, pins — or send us a part number and we will quote it.