Walk into any MRO shop and you'll find AN, MS, and NAS hardware mixed in the same bins. Plans call out all three. Suppliers stock all three. But the spec stamped on that bolt head isn't interchangeable with the other two — each system has a specific governing body, tolerance class, and application envelope. Getting this wrong means either over-engineering an airframe at unnecessary cost or under-engineering a joint that was designed for something better. This is the breakdown mechanics and EAB builders actually need, grounded in FAA AC 43.13-1B.
Why Three Standards Exist
AN (Air Force-Navy) hardware traces back to World War II joint procurement between the U.S. Army Air Forces and the Navy. The goal was a unified hardware specification that both services could order from the same manufacturers. AN standardization produced the bolt series most mechanics know by heart — AN3 through AN20, covering 10-32 through 3/4-16 thread sizes in alloy steel and aluminum. FAA AC 43.13-1B Chapter 7 calls out AN hardware by name throughout its structural repair guidance. If you're working on general aviation or building experimental, AN is the baseline you default to unless the drawing says otherwise.
MS (Military Standard) emerged from the Department of Defense's post-WWII consolidation of procurement standards. The intent was to fold the AN system into a unified DoD numbering system. Many AN part numbers have direct MS equivalents — the same physical part, the same drawing, a different designation. MS20365 is AN365. MS20033 is AN3. The cross-reference exists in NAVAIR 01-1A-8 and is definitive for AN/MS equivalency verification. Calling a part MS rather than AN does not make it stronger, more precise, or better documented. It means the DoD program that specified it used the MS numbering system.
NAS (National Aerospace Standard) is maintained by the Aerospace Industries Association (AIA) on behalf of airframe and engine OEMs. NAS standards are written to tighter tolerances than AN equivalents, cover higher strength grades, and address applications — close-tolerance shear joints, high-cycle structural joints — where the AN system's clearance-fit tolerances are inadequate. When an aircraft manufacturer engineers a close-tolerance shear joint, they specify NAS hardware because the tolerance class is part of the design.
Spec System Comparison
| Feature | AN | MS | NAS |
|---|---|---|---|
| Governing Body | Air Force-Navy Aeronautical Standards | Dept. of Defense | Aerospace Industries Association |
| Tensile Strength (alloy steel) | ~125,000 psi | Same as AN equivalent | ~160,000 psi (close-tolerance series) |
| Shank Tolerance | Clearance fit (±0.003") | Clearance fit | Close tolerance (±0.0005") |
| Traceability Required (FAA) | Yes, for certified aircraft | Yes | Yes |
| Primary Application | General structural, GA, homebuilts | Defense/production spec | High-load, precision-hole applications |
| Interchangeable? | Many MS bolts are AN equivalents | Per cross-reference only | No — different tolerance class |
When AN Is the Right Call
AN hardware covers the vast majority of general aviation and experimental homebuilt (EAB) structural applications. The AN3–AN20 bolt range — 10-32 thread through 3/4-16 — handles 95% of EAB airframe connections. Cabane struts, control surface hinges, wing attach fittings, firewall penetrations, landing gear attach — if the plans say AN, you use AN. No upgrade is warranted or appropriate without engineering review of the hole tolerance and joint design.
- FAA AC 43.13-1B Appendix A explicitly lists acceptable AN hardware for structural repairs on certificated aircraft
- The clearance-fit tolerance of AN bolts is designed to work with the drilled holes and close-tolerance bushings in typical GA airframe structure
- AN hardware is the most widely available, best-documented, and most cost-effective option for standard structural applications
- EAB builders: when plans specify AN, substituting NAS "because it's stronger" introduces a tolerance mismatch in holes drilled for AN clearance fit
When MS Applies
If your plans or maintenance manual calls out an MS part number, you order that MS part number — or verify its AN equivalent through the cross-reference index before substituting. MS hardware is common in military aircraft maintenance documentation, in production aircraft service manuals from the 1960s onward, and in parts lists for aircraft developed under DoD programs.
- MS20033 = AN3; MS20034 = AN4 (verify via NAVAIR 01-1A-8 before substituting in any certified application)
- MS20365 = AN365 (self-locking nut, all-metal prevailing torque); MS20310 = AN310 (castle nut)
- Verifying the cross-reference is not optional — there are MS numbers with no AN equivalent, and some cross-references have revision-level dependencies
- MS does not imply any strength, tolerance, or quality improvement over AN. The designation is administrative.
When NAS Is Required
NAS hardware enters the picture when the joint design depends on close-tolerance hole fit for shear load transfer. When an engineer machines a hole to ±0.0005" and specifies a NAS bolt, the structural calculation assumes that the bolt shank fills that hole — the load is transferred through bearing contact across the full shank diameter, not through friction or preload. Put an AN clearance-fit bolt in that hole and you've changed the load path. The joint will work at low load, and it will fail differently at high load than the engineer expected.
- NAS1303–NAS1320 series: Close-tolerance hex head bolts, 160,000 psi minimum tensile strength, drilled shank variants available for safety wire or cotter pin retention
- Primary structural wing attach bolts on high-performance homebuilts (RV-series, Lancair, Glasair) frequently call NAS hardware at spar attach points
- Main spar fittings, engine mount attach points, and other primary structural hard points in production aircraft are engineered for NAS close-tolerance fit
- Do not install NAS bolts in standard clearance-fit holes: the engineering benefit is derived from hole fit, not just the higher tensile strength of the bolt material
The Substitution Rule
The substitution rules are not guidelines — they are structural engineering constraints. The following applies regardless of whether you're building experimental or maintaining certificated aircraft:
- Never substitute AN for NAS in a close-tolerance hole without engineering sign-off that the joint analysis has been redone for clearance-fit behavior
- Never substitute NAS for AN in an application not designed for close-tolerance fit — the hole tolerance on the structure won't match, and you gain no benefit from the higher-strength hardware
- MS/AN crossover is legal with verified equivalency per the NAVAIR 01-1A-8 cross-reference index. Same part, different name.
- FAA Form 8130-3 is required for all three spec systems on certificated aircraft. New surplus AN, MS, and NAS hardware with 8130-3 from the original PAH is legally equivalent to new production hardware — see our guide on FAA 8130-3 explained for what the form actually certifies.
When a plan or manual specifies a part number, that number encodes the spec system, tolerance class, material, and geometry. The number is the design intent. If you don't understand why the engineer picked that spec, the answer is to ask — not to substitute based on what's in the bin.