Understanding MS Aircraft Parts and Part Numbers
MS part numbers identify parts created under the U.S. Military Standard system. This guide explains the history and purpose of MS standards, how MS part numbers are structured, what common series represent, and why every dash number and suffix must be checked against the controlling standard.
What Does “MS” Mean?
MS generally means Military Standard. An MS number identifies a standard drawing, specification sheet, or standardized part created for use across U.S. military departments and defense programs.
MS standards expanded the government’s common-part system beyond earlier AN hardware and helped organize fasteners, electrical components, seals, fittings, clamps, connectors, and many other products under controlled requirements. MS numbers remain common in military and civil aircraft manuals, illustrated parts catalogs, procurement systems, and maintenance records.
An MS Number Must Be Read with Its Standard
Unlike a simple catalog code, an MS number refers to a specific government standard or drawing. The number after “MS” identifies the document or part family—not a universal product category that can be decoded from memory.
- There is no single numbering formula that works for every MS family.
- The dash number may mean size, length, material, finish, class, style, or another family-specific option.
- Letter suffixes and revision status can change the exact item.
- Use the controlling standard or approved technical data to decode the complete number.
Why Military Standards Were Developed
MS documents supported common procurement and engineering requirements across military services, contractors, aircraft programs, and supply organizations.
Common Requirements
A single standard could define dimensions, materials, finishes, testing, inspection, marking, and performance.
Interchangeable Supply
Standardized parts could be sourced and stocked by a common number instead of relying only on proprietary manufacturer numbers.
Controlled Procurement
Military buyers and contractors could reference one document to communicate the exact product requirements.
Long-Term Support
MS numbers helped maintain aircraft, vehicles, electronics, and equipment across long service lives and multiple suppliers.
How an MS Part Number Is Usually Built
The exact structure depends on the standard. Many MS part numbers contain the following types of information, but their meaning and position are controlled by the individual MS document.
MS + STANDARD NUMBER + DASH OPTION + SUFFIX
This is a conceptual layout only. The actual decoder must come from the applicable MS standard, specification sheet, approved manual, or current cross-reference.
Common MS Series and Their Uses
MS numbering covers far more than fasteners. The examples below show familiar aviation product families and illustrate why the series number matters.
MS20004–MS20024 Bolts
Close-tolerance and high-strength bolt families used in aircraft structural applications. Exact diameter, length, grip, head, drilling, material, and finish depend on the specific standard and dash number.
- Structural and close-tolerance joints
- Precise fit and load-transfer applications
- Must not be replaced by a general-purpose bolt without approved data
MS21042 / MS21043 Nuts
Common self-locking nut families used throughout aircraft assemblies. Different series can identify reduced-height, all-metal, temperature-related, or material configurations.
- Vibration-resistant threaded joints
- Series-specific temperature and reuse limitations
- Thread size and material must be confirmed
MS20002 Washers
Hardened washer families commonly used with high-strength or close-tolerance fasteners where bearing strength and controlled dimensions matter.
- Load distribution under bolt heads and nuts
- Available in family-defined sizes and thicknesses
- Not interchangeable with every plain washer
MS24665 Cotter Pins
Standard cotter-pin families used as a mechanical locking device with castellated nuts, clevis pins, and other drilled hardware.
- Positive mechanical retention
- Diameter and length are encoded by the dash number
- Normally replaced after removal rather than reused
MS28775 O-Rings
A widely recognized packing and O-ring family used in aircraft fluid-system sealing applications. Dash numbers identify standardized sizes.
- Hydraulic, fuel, pneumatic, and related sealing uses
- Compound and fluid compatibility still require verification
- Size alone does not establish suitability
MS Electrical & Connector Families
Many MS series identify circular connectors, contacts, terminals, switches, relays, lamps, wire accessories, and other electrical components.
- Shell size, contact arrangement, keying, and finish may be encoded
- Mating compatibility must be verified
- Environmental class and material may vary by suffix
How to Decode an MS Number Correctly
Start with the complete number and locate the specific MS document. The document normally includes a table, figure, or part-identification section explaining how the base number, dash number, class, style, size, material, finish, or suffix are combined.
- Identify the exact MS document: For example, MS21042 and MS21043 are separate standards even though both cover self-locking nuts.
- Locate the part-identification section: This is where the document explains the complete part-number format.
- Decode every character: Do not stop after identifying the general product family.
- Check document status: The standard may be active, inactive for new design, cancelled, superseded, or replaced.
- Confirm the approved replacement: A superseding number must be evaluated against the aircraft or assembly data.
What Each Part of an MS Number Can Control
A short dash code or suffix can change the dimensions, performance, material, or compatibility of the item.
| Number Element | What It May Identify | Why It Matters |
|---|---|---|
| MS Series Number | The exact military standard drawing or product family. | A nearby MS number may cover a completely different configuration or product. |
| Dash Number | Size, diameter, length, thickness, pressure class, contact arrangement, or another option. | The meaning changes from one MS family to another. |
| Material Code | Steel, corrosion-resistant steel, aluminum alloy, elastomer compound, or another specified material. | Material affects strength, temperature, corrosion, fluid compatibility, and electrical performance. |
| Finish Code | Plating, passivation, anodizing, coating, color, or another protective finish. | Finish can affect corrosion resistance, bonding, torque, mating, and environmental suitability. |
| Class or Style | Locking method, head type, mounting style, sealing arrangement, environmental class, or design option. | Items in different classes may not be physically or functionally interchangeable. |
| Revision or Supersession | The current issue of the standard or the document that replaced it. | Legacy numbers may remain in manuals while procurement moves to a newer standard. |
MS Compared with AN
AN and MS are both government standard systems, but they came from different phases of military standardization. Many MS standards expanded, replaced, or supplemented earlier AN standards.
- AN means Army-Navy standard.
- MS means Military Standard.
- Both may appear in older and current aircraft records.
- A cross-reference should be verified before substitution.
MS, NASM, and SAE AS Standards
Some MS documents were later transferred, converted, or superseded by NASM or SAE Aerospace Standards as government standardization practices changed.
- Legacy manuals may continue to show the MS number.
- The current procurement number may use NASM or AS.
- Current document status should be checked in an official standards source.
- Supersession does not automatically authorize installation on every aircraft.
Similar MS Numbers Are Not Automatically Interchangeable
MS part numbers can look almost identical while representing different sizes, contact arrangements, materials, finishes, environmental classes, locking features, or performance requirements.
- Verify the complete MS number, including all dashes and suffixes.
- Confirm the standard’s current status and superseding document.
- Review dimensions, material, finish, class, and configuration.
- Do not substitute based only on appearance or a partial number.
Where MS Parts Are Used
- Airframe structural assemblies
- Flight-control and linkage systems
- Hydraulic, fuel, oil, and pneumatic systems
- Electrical wiring and connector systems
- Engines, accessories, and equipment mounting
- Sealing, locking, identification, and safety applications
What to Review Before Ordering
- Complete MS number, including every dash and suffix
- Current document status and superseding number
- Dimensions, class, style, material, finish, and configuration
- Condition, quantity, and unit of sale
- CoC, trace, manufacturer paperwork, or required test records
- Aircraft eligibility and approved substitution data
How to Search for MS Parts
Search the full MS part number first, including all leading zeros, dashes, and suffixes. If the exact search does not return a result, try the base MS document number or the same number without punctuation to locate related inventory.
Use related or superseding numbers as research leads—not automatic substitutes. For purchasing and installation decisions, the controlling standard, approved aircraft data, illustrated parts catalog, maintenance manual, engineering instruction, or other accepted technical record should take priority over a general online decoder.
Need Help Identifying an MS Part?
Send AVBOX US the complete MS number, aircraft application, dimensions, photographs, alternate number, superseding number, or documentation requirement you are working through.