How to Read NAS Part Numbers: A Practical Guide

NAS6204-12. NAS1351-3-16. NAS43DD3-8. If you buy aerospace hardware for a living, part numbers like these cross your desk every day — and every character in them is doing a job. A NAS part number is not a random SKU: it is a compressed description of a standard, a size, a material, and a set of optional features. Learn to unpack it and you can sanity-check a quote in seconds, catch a transcription error before it becomes a wrong shipment, and ask suppliers sharper questions.

This guide walks through the anatomy of NAS part numbers the way a distributor reads them at the quoting desk and on the receiving dock. It stays honest about the limits, too: dash-number semantics vary from one standard to the next, and the standard sheet — not a blog post — is always the final authority.

Where NAS Numbers Come From

The National Aerospace Standards are an industry standards series maintained by the Aerospace Industries Association (AIA) since the early 1940s. They sit alongside two older military families you will constantly see mixed into the same bill of materials: AN (Army-Navy) and MS (Military Standard) parts. In the 1990s, responsibility for many inactive military fastener standards moved to the AIA, which republished them as NASM documents — which is why an MS callout on an older drawing sometimes leads you to a NASM standard today.

As a rough generalization, NAS standards cover a large share of the higher-strength and closer-tolerance hardware in an airframe: high-strength bolts, precision screws, spacers, bushings, and specialty washers. That is one reason a NAS bolt and a dimensionally similar AN bolt are not the same part — more on that below.

The Anatomy of a NAS Part Number

Every NAS callout has up to three layers:

1. The prefix — NAS — identifying the standards series.
2. The base number — 6204, 1351, 1149 — which identifies the standard itself, and with it the part family: head style, thread form, general material system, and every fixed characteristic.
3. Dash numbers and letter codes, which select the variable characteristics: diameter, grip or length, material option, finish, self-locking features, drilled holes, and so on.

The catch is that the boundary between layers moves. In some families the base number itself encodes size — in the NAS6203–6220 bolt series, the last digits index the nominal diameter. In others, size lives entirely in the dash numbers — a NAS1351-3-16 socket head cap screw carries its thread size and length after the base number. This is why there is no single universal parsing rule: you learn the families you buy most, and you look up the rest.

Base Number Families Worth Knowing

A few ranges come up so often in fastener procurement that they are worth memorizing:

• NAS600–606 — aircraft machine screws, with each number in the range corresponding to a nominal diameter.
• NAS1101 — another frequently quoted machine screw series; as with most screw standards, size and length ride in the dash numbers.
• NAS1351 and NAS1352 — socket head cap screws, offered across multiple materials and locking configurations.
• NAS6203–6220 — hex head bolts; the series steps through nominal diameters as the base number increases.

Beyond threaded fasteners, the series covers plenty of the hardware around them: NAS43 spacers (often seen as NAS43DD callouts), NAS1149 washers, and NAS75 and NAS77 bushings, among many others. You can browse these families directly in our bolts and screws catalogs.

Dash Numbers: Diameter, Grip, and Length

Dash numbers are where most reading errors happen, because their meaning is family-specific. Some patterns are common enough to be useful defaults — as long as you treat them as defaults, not guarantees:

• For many NAS bolt standards, the dash number after the base expresses grip length in sixteenths of an inch. A -12, in those families, means a nominal grip of 12/16 — three-quarters of an inch.
• For many screw standards, the first dash number indexes thread size and the second gives length, again typically in sixteenths.
• Letters can interrupt or trail the dash numbers to add features — a drilled shank, a locking element, an alternate finish.

The increments are the trap. Sixteenths of an inch is the most common convention, but it is not a law of nature; some standards use other increments or express dimensions differently. When in doubt, pull the standard sheet and confirm.

Letter Codes: Material, Finish, and Features

Letter codes appear in three places: baked into the base callout (NAS43DD), between the base and dash number, or trailing the dash number. They typically select material, finish, or optional features. A few conventions recur across many — though not all — standards:

• C frequently designates corrosion-resistant steel (CRES) in place of the base alloy steel.
• The absence of any material letter usually means the base configuration of the standard — for many older bolt standards, cadmium-plated alloy steel.
• Trailing letters often add drilled cotter-pin holes, drilled heads for lockwire, or self-locking elements.

Treat every one of these as a hypothesis to verify. The same letter can mean different things in different families, and some codes — like the DD in NAS43DD — are simply part of the base callout rather than an option you can drop. If a letter is in the callout on your drawing, it belongs in the callout on your purchase order, verbatim.

A Worked Example: NAS6204-12

Put the pieces together:

1. NAS — the part is defined by a National Aerospace Standard published by the AIA.
2. 6204 — the base number places it in the NAS6203–6220 hex head bolt family. Within this series the final digits index nominal diameter, and 6204 corresponds to the 1/4-inch, fine-thread member of the family.
3. -12 — for this family, the dash number typically expresses grip length in sixteenths of an inch, giving a nominal grip of 3/4 inch. Confirm against the current standard sheet before ordering.
4. No letter codes — the callout selects the base material and finish, with no drilled holes or locking features added.

Now reverse the exercise. If a requisition reads “NAS6204-12H” or “NAS6204C12”, the extra letter should stop you cold: it changes the part — drilled features, different material — and you need to resolve exactly what it means before quoting. One letter is never noise.

Common Pitfalls

• Superseded and inactive standards. Standards get revised, canceled, and replaced — including wholesale conversions like the MS-to-NASM transfer. If a drawing calls an old standard, buy exactly what your engineering authority requires and check the standard's current status rather than silently substituting its successor.
• Lookalike AN, MS, and NAS numbers. An AN4 and a NAS6204 are both 1/4-28 hex bolts at first glance, but they differ in material strength, tolerances, and markings. They are not interchangeable unless the engineering authority for your application says so in writing.
• Dropped characters. A missing letter or a transposed digit produces a real, orderable — and wrong — part number more often than you would hope. Copy callouts character-for-character, and have receiving verify part markings against the purchase order, not against memory.
• Assuming dash semantics transfer. The -10 that means grip in one family can mean overall length, or something else entirely, in another. Parse each family on its own terms.
• Ordering by description. “Quarter-inch NAS hex bolt, 3/4 grip” describes several different part numbers. The callout is the contract; descriptions are commentary.

Part number literacy is half of buying hardware well; the other half is making sure the parts that arrive are what the paperwork says they are. That is a quality-system problem — see our guide to AS9120 vs AS9100 for what a distributor's certification actually covers.