Understanding AN Aircraft Parts and Part Numbers
AN hardware helped create a common language for military and civil aircraft maintenance. This guide explains where AN standards came from, how AN part numbers are structured, what the major number families generally represent, and why every dash number and suffix must be verified before ordering.
What Does “AN” Mean?
AN refers to the historic Army-Navy aircraft standardization system. The purpose was to create common specifications and identifying numbers for hardware and components that could be used across military aviation programs instead of relying on a different proprietary part for every aircraft or service branch.
AN numbers became widely recognized throughout aviation and remain common in maintenance manuals, illustrated parts catalogs, procurement records, stockrooms, and legacy aircraft documentation. Many original AN standards were later revised, superseded, or cross-referenced to MS, NAS, NASM, AS, SAE, or other standards, but the AN designation remains deeply embedded in aviation.
AN Is a Standard Family—not a Complete Description
The letters “AN” tell you that the item belongs to an Army-Navy standard family. They do not, by themselves, tell you the exact diameter, length, thread, material, finish, drilling, configuration, or current superseding standard.
- Always use the complete part number.
- Decode the number using the correct standard for that specific family.
- Do not apply the numbering rules for an AN bolt to an AN nut, washer, fitting, or screw.
- Confirm whether the number has been superseded or cross-referenced.
Why AN Standards Were Created
Standardization made it easier for aircraft designers, military services, maintenance organizations, and suppliers to identify and procure consistent hardware across many aircraft types.
Common Identification
A shared number gave engineers, maintainers, and buyers a consistent way to identify an item.
Interchangeable Supply
Standard requirements reduced dependence on a single aircraft manufacturer’s proprietary hardware number.
Controlled Requirements
The applicable standard defined dimensions, materials, finishes, performance, inspection, and other requirements.
Fleet Support
Standardized hardware simplified procurement, stocking, repair, and long-term support across aircraft fleets.
How an AN Part Number Is Usually Built
There is no single decoder that works for every AN product. Each standard family establishes its own numbering rules. However, many AN part numbers contain the same general types of information.
AN + FAMILY + DASH SIZE + SUFFIX
This is a conceptual pattern only. The position and meaning of each character must be checked against the applicable AN standard or an approved reference for that exact family.
Common AN Series and Their Uses
The examples below show well-known AN families. They are intended as an orientation guide, not as a substitute for the controlling standard or aircraft maintenance data.
AN3–AN20 Bolts
A widely recognized family of aircraft machine bolts. The family number relates to nominal diameter, while the dash portion commonly identifies length. Suffixes may identify drilled or undrilled configurations and material options.
- Structural and general aircraft fastening
- Used with nuts, washers, and locking devices
- Exact grip and thread relationship must be verified
AN23–AN30 Clevis Bolts
Clevis-style bolts are commonly associated with shear-loaded joints and control-system connections where the threaded portion is not intended to carry the primary shear load.
- Control linkages and clevis joints
- Typically identified by diameter and length details
- Not a substitute for a standard bolt without approved data
AN42–AN49 Eye Bolts
Eye-bolt families provide an integral eye or loop for attachment, tension, linkage, or control applications.
- Cable, linkage, and attachment uses
- Thread and shank dimensions vary by family
- Load direction and application must be verified
AN310–AN320 Nuts
AN nut families include castellated, plain, jam, self-locking, and other configurations. The series identifies the basic nut type; dash numbers and suffixes identify size and configuration.
- Used with bolts, studs, and threaded fittings
- Locking method varies by series
- Temperature and reuse limitations may apply
AN900-Series Washers
AN washer families include plain washers and specialized washer configurations used to distribute load, adjust spacing, protect surfaces, or support locking arrangements.
- Plain, thin, special, and sealing-related types
- Inside diameter and thickness matter
- Washer selection is application-specific
AN Fittings and Fluid Hardware
AN fluid-system families include fittings, elbows, tees, adapters, nipples, plugs, and related hardware used in aircraft fuel, oil, hydraulic, pneumatic, and instrument systems.
- Tube size and thread form must be confirmed
- Material and finish may change with suffix
- Pressure and fluid compatibility must be verified
Example: Understanding a Typical AN Bolt Number
AN bolt families are often used to teach AN numbering because their structure is relatively familiar. A typical bolt number may contain:
- AN: Army-Navy standard prefix.
- Diameter family: The number immediately after AN commonly identifies nominal bolt diameter in sixteenths of an inch for the familiar AN3 through AN20 machine-bolt families.
- Dash length: The dash number commonly identifies bolt length in eighth-inch increments, subject to the rules of that standard.
- Configuration suffix: A suffix may identify an undrilled shank, corrosion-resistant material, or another family-defined option.
This logic is useful for AN machine bolts, but it should not be blindly transferred to nuts, washers, fittings, screws, or other AN families. Their series and dash codes follow different rules.
What Each Part of the Number Can Change
A one-character difference can produce a physically different or operationally unsuitable part.
| Number Element | What It May Control | Why It Matters |
|---|---|---|
| Series Number | Basic product family, head style, nut style, washer type, fitting geometry, or other standard design. | A nearby series may be a completely different product, not merely another size. |
| Diameter Code | Nominal shank, thread, tube, or hole size depending on the family. | Incorrect diameter can prevent assembly or reduce joint strength. |
| Length or Grip Code | Overall length, grip length, threaded length, or another dimension. | Correct grip placement is important in structural joints and control linkages. |
| Material Suffix | Alloy steel, corrosion-resistant steel, aluminum alloy, brass, or another permitted material. | Material changes strength, corrosion resistance, temperature capability, and compatibility. |
| Finish or Plating | Cadmium plating, passivation, anodizing, or another protective finish when defined. | Finish affects corrosion resistance, electrical bonding, torque, and environmental suitability. |
| Drilling or Locking Option | Drilled head, drilled shank, castellated feature, self-locking feature, or another retention method. | The locking method must match the approved assembly design. |
AN, MS, NAS, and NASM
Aviation hardware standards evolved over time. Many older AN documents were superseded or replaced by Military Standard (MS), National Aerospace Standard (NAS), NASM, SAE Aerospace Standard (AS), or other documents.
- Legacy manuals may continue to list the AN number.
- A current product may carry a superseding MS, NAS, or AS number.
- Cross-reference does not always mean unrestricted interchangeability.
- Use current approved data to confirm the acceptable replacement.
Why AN Numbers Are Still Used
AN numbers remain common because aircraft fleets, manuals, inventory systems, and procurement records often remain in service for decades.
- Large installed base across civil and military aircraft
- Long-lived maintenance manuals and illustrated parts catalogs
- Established distributor and maintenance vocabulary
- Cross-reference value for newer standard numbers
Similar AN Numbers Are Not Automatically Interchangeable
Two AN numbers can look nearly identical while representing different dimensions, materials, finishes, drilling, locking methods, or product families.
- Verify the exact number from the applicable manual or approved record.
- Confirm the current standard and any superseding number.
- Review dimensions, material, finish, and configuration.
- Do not substitute based only on appearance or a partial number.
Where AN Hardware Is Used
- Airframe structural joints
- Flight-control linkages
- Engine and accessory installations
- Fuel, oil, hydraulic, and pneumatic systems
- Electrical bonding and equipment mounting
- Interior, fairing, access-panel, and support assemblies
What to Review Before Ordering
- Complete AN number, including every dash and suffix
- Current or superseding specification
- Diameter, thread, length, grip, and configuration
- Material, finish, condition, and unit of sale
- CoC, trace, manufacturer paperwork, or other required documentation
- Aircraft eligibility and approved substitution data
How to Search for AN Parts
Search the complete AN part number first. When a result is not found, try the same number with and without spaces or dashes, then search the base family number. Review related or superseding numbers only as leads until they are verified.
For purchasing and installation decisions, the controlling standard, approved aircraft data, illustrated parts catalog, maintenance manual, engineering instruction, or other accepted record should take priority over a general online decoder.
Need Help Identifying an AN Part?
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