How to Read MS (Military Standard) Part Numbers: A Practical Guide

MS15795-810. MS21042-3. MS20470AD4-4. If you buy aerospace hardware for a living, MS callouts like these move across your desk every day, and each character is carrying information. An MS part number is not an arbitrary SKU: it is a compressed reference to a military standard, and through it a family, a size, a material, and a finish. Learn to unpack it and you can sanity-check a quote in seconds, catch a transcription slip before it turns into a wrong shipment, and ask suppliers sharper questions.

This guide walks through the anatomy of MS 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, many older MS standards have been re-designated, and the standard sheet, not a blog post, is always the final authority.

Where MS Numbers Come From

The Military Standards are a US Department of Defense series of standard parts and design details. On the fastener and hardware side, MS numbers grew up alongside two families you will constantly see mixed into the same bill of materials: AN (Army-Navy) parts, the older family that MS numbering gradually extended and supplemented, and NAS (National Aerospace Standard) parts, the industry series published by the Aerospace Industries Association. The three systems coexist, cross-reference each other, and are not interchangeable unless an engineering authority says so.

The single most important piece of context for reading MS callouts today is what happened when the DoD moved responsibility for many inactive standards to industry. A large share of the older MS fastener and hardware standards were transferred and re-designated as NASM documents, republished by the AIA. NASM here stands for the National Aerospace Standard version of the old military document, and in practice an MS number and its NASM successor typically describe the same part, for example MS20470 and NASM20470. That is why an MS callout on an older drawing often leads you to a current NASM standard when you go looking for the controlling sheet.

The Anatomy of an MS Part Number

Most MS hardware callouts have up to three layers:

1. The prefix, MS, identifying the part as defined by a Military Standard.
2. The base number (15795, 21042, 20470), which identifies the standard itself, and with it the part family: type, head or profile style, general material system, and every fixed characteristic the standard nails down.
3. Dash numbers and letter codes, which select the variable characteristics: diameter, length or grip, material option, finish, and features such as locking elements.

The catch is that the boundary between layers moves from standard to standard. In some families the base number is the whole story for the fixed part and every dimension lives in the dash numbers. In others a letter is baked into the callout to select an alloy before you ever reach a dash. This is why there is no single universal parsing rule for MS numbers: you learn the families you buy most, and you look up the rest against the controlling standard.

Base Number Families Worth Knowing

A handful of MS base numbers come up so often in fastener and hardware procurement that they are worth recognizing on sight:

• MS20470 and MS20426: solid rivets, universal head and 100-degree countersunk head respectively, both widely re-designated to NASM equivalents.
• MS21042 and MS21044: self-locking nuts, quoted across multiple materials and temperature ratings.
• MS15795 and MS15795-8xx: flat washers, a family where the dash number selects the specific washer size.
• MS35206 and MS35207: machine screws, with size and length carried in the dash numbers.

You will find these and thousands of neighboring MS families in our rivets, washers, and bolts catalogs. With more than 9,500 MS parts in the catalog, recognizing the base family is the fastest way to narrow a search.

Dash Numbers: Size, Length, and Grip

Dash numbers are where most reading errors happen, because their meaning is set by the individual standard, not by a global rule. Some patterns recur often enough to be useful defaults, as long as you treat them as defaults, not guarantees:

• For many MS screw and rivet standards, a first dash number indexes nominal diameter and a second gives length, frequently in sixteenths of an inch.
• For washer and nut families the dash number often selects the whole size directly, keyed to a screw or bolt series rather than a raw measurement.
• Letters can interrupt or trail the dash numbers to add a material or feature, for example an AD in MS20470AD4-4 that designates the rivet alloy.

The increment and the reference point are the traps. Sixteenths of an inch is a common convention, but it is not a law of nature, and a dash that reads as a size code in one family reads as a length in another. When in doubt, pull the standard sheet, or its NASM successor, and confirm.

Letter Codes: Material, Finish, and Features

Letter codes appear between the base and dash number, trailing the dash number, or occasionally baked into the base callout. They typically select material, finish, or an optional feature. A few conventions recur across many, though not all, standards:

• C frequently designates corrosion-resistant steel (CRES) in place of the base alloy in nut and bolt families.
• AD, D, and DD commonly designate specific aluminum rivet alloys in the MS20470 and MS20426 families.
• The absence of any material letter usually selects the base configuration the standard defines by default.

Treat every one of these as a hypothesis to verify. The same letter can mean different things in different families, and some letters are part of the 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.

Worked Examples: MS15795-810 and MS20470AD4-4

Put the pieces together with a washer callout first:

1. MS: the part is defined by a Military Standard, one that may today be controlled by a NASM successor document.
2. 15795: the base number places it in the flat washer family. The standard fixes the material system and the general form of the washer.
3. -810: for this family the dash number selects a specific washer size within the standard's size table, rather than expressing a raw dimension you can compute in your head. Confirm the exact bore and outside diameter against the current standard sheet before ordering.

Now a rivet callout, MS20470AD4-4:

1. MS20470: the universal head solid rivet family, widely re-designated as NASM20470.
2. AD: a material code designating a specific aluminum rivet alloy; changing it changes the part.
3. 4-4: for this family the first figure typically indexes nominal diameter in thirty-seconds of an inch and the second gives length, but confirm the increments against the controlling standard rather than assuming.

Reverse the exercise as a habit. If a requisition drops the AD from the rivet, or reads MS15795-8 where the drawing says MS15795-810, the missing characters should stop you cold. They change the part, and you need to resolve exactly what the drawing intends before quoting. One character is never noise.

Common Pitfalls

• MS-to-NASM re-designation. Many MS fastener and hardware standards were transferred to industry and republished as NASM documents, and some MS numbers were canceled outright. When a drawing calls an old MS standard, confirm its current status and which document actually controls the part today rather than assuming the MS sheet is still live.
• Lookalike AN, MS, and NAS numbers. The three systems reference one another and share dimensions, so parts from different families can look almost identical on a shelf while differing in material, tolerance, or 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. A dash that selects a size code in a washer family can mean length in a screw family and diameter in a rivet family. Parse each MS standard on its own terms.
• Ordering by description. A quarter-inch flat washer or a universal head aluminum rivet describes many different MS part numbers. The callout is the contract; descriptions are commentary.

If the MS-versus-NAS relationship is what you are untangling, our companion guide to reading NAS part numbers covers the same NASM re-designation story from the NAS side. When the question is which fastener a bolt callout is actually specifying, see shear vs tension bolts, and for OEM-specific numbering, our guide to Boeing BAC part numbers.

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.