MS, NAS, and AN Fastener Numbering Systems Explained

A drawing calls out MS21042L3, or a customer's purchase order lists AN960-10L, and somebody at the counter has to figure out what that actually means before they can pull stock or place an order. If you work with military and aerospace hardware even occasionally, you'll run into three overlapping numbering systems — AN, MS, and NAS — and they don't follow one consistent format. This guide breaks down how each system is built, walks through real part numbers so you can decode them yourself, and explains why you'll often see an AN number and an MS number pointing at the same physical part.

Three Standards, One Hardware Aisle

AN, MS, and NAS all describe fasteners built to military and aerospace tolerances, but they came from different places and different eras. Knowing which family a part number belongs to tells you a lot about the part before you even look up the dimensions.

AN — Army-Navy

AN is the oldest of the three. The designation stands for Army-Navy, a joint standard adopted before the services consolidated their hardware specs. Most AN numbers date to the World War II era and cover the basic airframe hardware still flying today — bolts, washers, cotter pins, rivets, and clevis pins. AN part numbers are short: a letter/number series prefix followed by dash numbers for size.

MS — Military Standard

MS stands for Military Standard. The MS system took over much of the AN catalog starting in the 1950s as the services standardized hardware across branches, and it's the system you'll see cited most often on modern drawings. MS covers the same general categories as AN — bolts, nuts, washers, pins — plus categories AN never had, like self-locking nuts and electrical hardware. MS numbers are usually five digits following the prefix (MS21042, MS35338) with a dash number or suffix for size and material.

NAS — National Aerospace Standard

NAS stands for National Aerospace Standard, maintained by the Aerospace Industries Association rather than the Department of Defense. NAS parts generally cover higher strength grades, closer tolerances, or configurations that MS and AN don't address — high-strength structural bolts, close-tolerance washers, and specialty pins used in primary structure. If a drawing calls for NAS hardware instead of the equivalent MS part, it's usually because the application needs the tighter tolerance or higher strength NAS provides.

Quick Reference: Which Family Is It?

How to Decode a Part Number

Every part number in these systems follows the same basic logic: a prefix identifies the part family and configuration, then a dash number (or attached suffix) tells you the diameter, length, material, or finish. The exact format varies by series, so the best way to learn it is by working through real examples.

Example 1: AN3 Bolt Series

AN3 through AN20 covers standard hex-head and clevis airframe bolts, sized by shank diameter in sixteenths of an inch. AN3 is a 3/16″ diameter bolt (10-32 thread), AN4 is 1/4″ (1/4-28 thread), AN5 is 5/16″, and so on up the series. After the base number, a dash number specifies grip length, and letter suffixes modify the head or shank.

So AN3-5A reads as: 3/16″ diameter airframe bolt, grip length code 5, undrilled shank.

AN Bolt Series — Base Number, Diameter, and Thread Size (AN3–AN16)

The base number (before the first dash) is constant for a given bolt diameter across the whole AN3–AN20 hex-head and clevis bolt family. Use this table to go straight from a base number to the thread you'll be ordering nuts and washers for:

Sources cross-checked: Zenith Aircraft Company AN Bolt Dimensions reference; Pegasus Auto Racing Supplies AN bolt torque/identification reference (FAA AC 43.13-1B basis); Pegasus and Spencer Aircraft product listings for AN9–AN16. Note that AN9 and AN11 are not standard production sizes in most current supplier catalogs — AN9 (9/16-18) does exist in the original spec and some supplier lines, but AN10, AN12, AN14, and AN16 are the sizes you'll most commonly find stocked above 1/2″.

Example 2: AN960 Flat Washer

AN960 is the standard flat washer for airframe bolts, and it's one of the most common part numbers you'll see on any aircraft or ground-support hardware list. The dash number after AN960 gives the bolt size the washer fits, and an “L” suffix flags the light (thin) version.

AN960 Dash Number to Dimension — Common Sizes

Below dash-10 (the #2 through #10 machine-screw sizes), the dash number is the screw number itself. From dash-416 up, the first digits are the bolt diameter in sixteenths and the trailing “16” is a fixed series marker, not a thickness value — the “L” suffix is what actually flags the thinner light-series washer. Dimensions below are nominal, from a stainless AN960 dash chart; standard-series steel and aluminum washers share the same ID/OD and use the same nominal thickness pattern.

Sources cross-checked: Southern Fasteners AN960 stainless flat washer dimension chart; individual part listings from Aircraft Spruce and Military Fasteners for AN960-10, -416, -516, -616, and -816 dash numbers, confirming ID/OD/thickness against the same nominal values. Always confirm max/min tolerance bands against the current AN960 spec sheet before finalizing a design dimension — the values above are nominal reference figures.

Example 3: MS35338 Split Lock Washer

MS35338 is the split (helical spring) lock washer used under bolt heads and nuts on general airframe and ground equipment hardware. The dash number specifies the bolt size the washer is sized for.

MS35338-42 reads as: split lock washer, size 42 in the MS35338 dash chart, sized for a specific bolt diameter (look up the exact bolt size against the MS35338 dash table for your revision, since dash-to-size mapping is standard-specific).

Example 4: MS21042 Self-Locking Nut

MS21042 is an all-metal, self-locking, reduced-hex nut — no nylon insert, the locking action comes from a deformed thread section instead. It's rated for higher temperatures than nylon-insert equivalents because there's no plastic to degrade. The dash number gives the thread size, and a material letter can follow.

MS21042L3 reads as: MS21042 series self-locking nut, “L” material/finish designator, dash-3 thread size (10-32). See our companion guide, Self-Locking Aerospace Fasteners, for the full breakdown of MS21042 vs MS21044 and where each is used.

MS21042 Dash Number to Thread Size

Sources cross-checked: Military Fasteners individual part datasheets for MS21042-04, MS21042-06, MS21042-08, MS21042-3, MS21042-5, and MS21042-6 (each confirms thread size in both the product description and the technical datasheet); Aviation Parts Inc. product listings for MS21042-02 and MS21042-06 corroborating the same thread-size pattern. Note the dash-numbering isn't perfectly sequential with leading zeros for the smaller sizes (-04, -06, -08) versus no leading zero for -3 and up — both conventions appear in supplier catalogs for the same parts, so match on the full part number rather than the dash digit alone.

Example 5: NAS1149 Flat Washer

NAS1149 is a close-tolerance flat washer used where AN960 doesn't quite fit the requirement — typically where a tighter outside-diameter or thickness tolerance is called out on the drawing. Same basic idea as AN960: the dash number ties to bolt size, and a suffix can indicate material or thickness variant.

NAS1149FN832P reads as: NAS1149 flat washer, “FN” series designator, “8” for bolt size (1/2″), “32” for outside diameter code, “P” for passivated corrosion-resistant steel. NAS part numbers pack more information into the string than AN or MS, so when in doubt, cross-reference against the manufacturer's dash chart rather than guessing at segment boundaries.

How the NAS1149 String Is Built

Every NAS1149 part number follows the same general pattern: base number, then a material letter, then a bolt-size digit, then a thickness code, then a finish letter. The exact letters in play:

Three verified examples from the governing spec sheet and supplier catalogs:

• NAS1149FN832P — carbon steel washer (F), size/thickness code N832, cadmium plated (P). Used as the #8-size, 1/32″-thick washer commonly cross-referenced against AN960-8 hardware.
• NAS1149F0332P — carbon steel washer (F), size/thickness code 0332 (#10 screw size, 1/32″ thick), cadmium plated (P).
• NAS1149F0563P — carbon steel washer (F), size/thickness code 0563 (5/16″ bolt size, 1/16″ thick), cadmium plated (P).

Sources cross-checked: NAS1149 specification sheet excerpts (aaestore.com.au, Scribd-hosted NAS1149 spec copies) for the material/finish letter meanings; Military Fasteners part listings for NAS1149FN832P, NAS1149F0332P, and NAS1149F0563P confirming size and finish against the decoded string. Because the exact digit-grouping convention for the size/thickness segment varies across revisions of the spec, treat the general pattern above as the reliable takeaway and always confirm an exact dash-to-dimension match against the current NAS1149 dash chart or the supplier's part listing before ordering or substituting.

Dimensional Data Disclaimer. The dash-number, thread-size, and dimensional values in the tables above are compiled from published aircraft hardware supplier catalogs, dash charts, and part datasheets, cross-checked against at least two independent sources per table. They are provided as a quick-reference aid for identifying and quoting hardware. They are not a substitute for the controlling standard document (the AN, MS, or NAS spec sheet for that part number). For flight-critical, DFARS-relevant, or quality-system-controlled work, verify dimensions and thread sizes against the current official spec revision before designing, stocking, or certifying to them. Eugene Fastener provides this reference as a convenience and does not warrant these values against any specific drawing, revision, or application.

What Dash Numbers Actually Mean

The dash number is where most of the confusion happens, because it doesn't mean the same thing across every series. In general terms:

• On bolts (AN3, AN4, etc.): the dash number is a grip length code, not a raw dimension. You have to look up the dash-to-length conversion for that specific bolt series — it's not simply eighths or sixteenths of an inch across every series.
• On washers (AN960, NAS1149): the dash number ties to the bolt size the washer is meant to fit, not the washer's own dimensions directly.
• On nuts (MS21042, MS21044): the dash number is the thread size code — dash-3 is 10-32, dash-4 is 1/4-28, and so on up the standard thread progression used across MS hardware.
• Letter suffixes and prefixes layered onto the dash number typically call out material (cadmium-plated steel, stainless, aluminum) or a dimensional variant (light series, self-locking, drilled/undrilled).

The practical takeaway: never assume a dash number is a plain measurement. Pull the actual standard sheet or manufacturer's dash chart for that specific part family before you quote or stock against it. A transposed dash number on an MS or NAS part is an easy way to ship the wrong thread size.

Why You See Both AN and MS Numbers on the Same Drawing

Here's where a lot of first-time buyers get tripped up. Many AN part numbers were officially superseded by MS equivalents decades ago, but the AN callouts never fully disappeared from circulation. Older drawings, legacy tooling, and parts catalogs still reference the AN number even though the current standard is the MS version of the same physical part. In practice this means:

• A drawing revision from the 1970s might call out AN960-10, while a current bill of material for the same assembly calls out the MS equivalent. Both are asking for the same washer.
• Manufacturer catalogs and cross-reference charts will often list the AN number in parentheses next to the current MS number precisely because so much documentation in the field still uses the old designation.
• When you're quoting a job and see an AN number, it's worth checking whether an MS supersession exists before assuming it's a discontinued or hard-to-source part — it may simply be an old designation for a part that's still in full production under a different number.
• Not every AN part has an MS successor. Some AN hardware, especially in the rivet and basic pin categories, is still procured under its original AN designation because no MS supersession was ever issued.

If you're not sure whether a part number on your drawing is current, superseded, or cross-referenced to another family, send it to us. We check supersession status as part of quoting so you're not stuck sourcing an obsolete number when a direct, in-production equivalent exists.

Frequently Asked Questions

What's the difference between an AN bolt and an MS bolt?

In many cases they're the same part under two names — the MS number is the current designation and the AN number is the legacy one it superseded. Where they differ, MS bolts typically reflect updated material, tolerance, or marking requirements adopted after the original AN spec was written. Always check the specific series rather than assuming AN and MS are always interchangeable.

Is NAS hardware always stronger than MS hardware?

Not always, but often. NAS parts frequently fill a gap where the application needs tighter tolerance, higher strength, or a configuration that MS and AN don't cover. Some NAS numbers exist simply because the part geometry (a specific washer thickness, for example) isn't in the MS catalog at all, not because it's a higher-strength substitute.

Can I substitute an AN part for the MS part called out on a drawing?

Only if the AN part is the direct predecessor of that MS number and your quality system or customer specification permits it. Don't assume; check supersession documentation or ask your quality engineer. On DFARS-relevant or flight-critical work, get it in writing before substituting.

Why does the dash number on my nut not match the dash number on my washer for the same bolt?

Because dash numbering isn't standardized across series — each part family (bolts, nuts, washers) has its own dash chart. A 1/4-28 bolt, its matching nut, and its matching washer can easily carry three completely different-looking dash numbers even though they're built for the same joint.

Where do I find the official dash chart for a specific MS or NAS number?

The controlling standard document (available through the Defense Logistics Agency or ASSIST database for MS specs, and through the Aerospace Industries Association for NAS specs) has the authoritative dash chart. Manufacturer catalogs reproduce these charts and are usually faster to reference for day-to-day quoting.

What thread size is an AN4 bolt?

An AN4 bolt is 1/4″ nominal shank diameter with a 1/4-28 thread. The AN base number gives you shank diameter directly in sixteenths of an inch (AN4 = 4/16″ = 1/4″), and each base number in the AN3–AN16 series has one fixed thread size that goes with it — see the AN bolt series table above for the full list.

What does the dash number on an MS21042 nut mean?

On MS21042, the dash number is the thread size code, not a length or a material designation. MS21042-3 is 10-32, MS21042-4 is 1/4-28, MS21042-5 is 5/16-24, and so on up the standard thread progression — see the MS21042 dash-to-thread table above. A letter prefix on the dash (like the “L” in MS21042L3) calls out a material or finish variant of that same thread size, not a different size.

Related Resources

• Self-Locking Aerospace Fasteners — jet nuts, nut plates, and prevailing-torque locking elements
• Mil-Spec Coatings and Finishes for Fasteners — black oxide, cadmium, zinc-nickel, and passivation compared
• Aerospace & Mil-Spec Fasteners — capability overview
• Mil-Spec Certification & Paperwork Guide — what certs and traceability documents to expect
• DFARS-Compliant Fasteners Guide — domestic sourcing and specialty metals compliance