NAS vs. MS vs. AN Hardware: Aerospace Fastener Selection & Cross-Reference Guide

The primary difference between AN (Air Force-Navy), MS (Military Standard), NAS (National Aerospace Standard), and AS (Aerospace Standards) hardware lies in their governing authorities, manufacturing tolerances, tensile strength ratings, and application limits.

AN defines legacy standard structural specs (approx 125,000 psi), MS consolidates multi-branch defense hardware, NAS/NASM covers close-tolerance high-fatigue structural joints (160,000 to 180,000+ psi), and AS represents modernized commercial standards governed by SAE International.

Historical Eras & Evolution of Aerospace Fastener Standards

Aerospace hardware standards evolved across four major military and industrial eras:

• MS Standard (Military Standard): Introduced in the 1950s under the U.S. Department of Defense to unify procurement across all military branches. The MS catalog expanded beyond standard bolts to include self-locking nuts, electrical hardware, and specialized fittings.
• NAS & NASM Standard (National Aerospace Standard): Managed by the Aerospace Industries Association (AIA). NAS hardware addresses high-load, high-speed, and flight-critical structural airframe areas using high-fatigue materials, close-tolerance shanks, and reduced internal-wrenching drive heads.
• AS Standard (Aerospace Standards): Maintained and governed by SAE International (Society of Automotive Engineers). AS specifications represent modern commercial technical standards that replaced many canceled military hardware standards following the 1994 DoD Acquisition Reform.

Core Comparison: Mechanical Properties, Tolerances & Applications

• AN Hardware: Governed by Legacy Military / SAE. Features standard Class 3A fit shank tolerances and typical tensile strength around 125,000 psi. Applied in general airframe construction, non-critical brackets, and legacy aircraft maintenance.
• MS Hardware: Governed by Defense Logistics Agency (DLA) / DoD. Features standard to medium shank tolerances and tensile strength between 125,000 and 140,000 psi. Applied in standard military assemblies, non-critical engine systems, and interior structures.
• NAS / NASM Hardware: Governed by Aerospace Industries Association (AIA). Features tight, close-tolerance shanks (+0.000 / -0.0005 inches) and high tensile strength ranging from 160,000 to 180,000+ psi. Applied in flight-critical joints, wing attachments, high-fatigue points, and turbine engines.
• AS Hardware: Governed by SAE International. Covers high-performance civil and defense hardware, specialized fluid fittings, locking rings, and modernized replacements for legacy military spec sheets.

Decoding Aerospace Part Numbers: Structural Logic Breakdown

Aerospace part numbers encode exact dimensions, shank features, head designs, and alloys directly within the alphanumeric string.

AN Part Numbering Logic

(Example: AN4-12A vs. AN4H-12)

• AN: Prefix denoting Air Force-Navy standard.
• 4: Nominal shank diameter in 16ths of an inch (4/16 = 1/4 inch).
• H: Designates a drilled head for safety wire installation (e.g., AN4H-12).
• -12: Dash number for length, where the first digit represents whole inches and the second digit represents 8ths of an inch (1 inch + 2/8 inch = 1-1/4 inches total length).
• A: Designates an undrilled (solid) shank for use with self-locking nuts. Omitting the A indicates a drilled cotter pin hole in the shank.

MS Part Numbering Logic

(Example: MS21042-4 & MS20470AD4-6)

• MS21042: Specific specification sheet for lightweight, high-strength self-locking hex nuts.
• -4: Size suffix designating thread diameter (4/16 = 1/4 inch).
• MS20470: Standard for universal head solid aluminum rivets.
• AD: Material alloy code (2117-T4 aluminum alloy).
• 4: Rivet shank diameter in 32nds of an inch (4/32 = 1/8 inch).
• -6: Rivet length in 16ths of an inch (6/16 = 3/8 inch).

NAS Part Numbering Logic

(Example: NAS6604-12)

• NAS: National Aerospace Standard prefix.
• 6604: Specification code indicating a 160,000 psi tensile strength, close-tolerance alloy steel bolt (the digit 4 specifies a 1/4 inch nominal shank diameter).
• -12: Defines maximum grip length in the 16ths of an inch (12/16 = 3/4 inch). NAS dash numbers measure precise structural grip length rather than total bolt length.

Engineering Selection Criteria: Load, Environment & Fit

Tensile & Shear Fatigue Load Requirements

Joints subjected to continuous flight vibration require close-tolerance NAS or AS bolts (such as the NAS6604 series). Their precision shank fit eliminates internal joint play, reducing hole elongation, fretting corrosion, and structural fatigue failure.

Material & Galvanic Corrosion Compatibility

Prevent galvanic action by matching fastener alloy plating with base materials:

• Aluminum Structures: Cadmium-plated alloy steel (AMS-QQ-P-416) AN/MS/NAS hardware or passivated CRES stainless steel.
• Titanium Structures: Titanium alloy (6Al-4V) or A286 CRES hardware. Cadmium-plated fasteners must be avoided on titanium assemblies due to liquid/solid metal embrittlement risks.
• High-Temperature Engine Zones: Inconel 718 or A286 CRES NAS/AS fasteners rated for operating environments above 1,200 degrees Fahrenheit (649 degrees Celsius).

Aerospace Cross-Referencing & Supersession Guidelines

The MS to NASM & SAE AS Transition

Following the 1994 military spec cancellation initiative (Perry Memo), standard MS military specs converted into commercial registry systems:

• Active MS standard callouts transferred directly to NASM numbers managed by AIA, preserving identical part numbering (e.g., MS20470 maps directly to NASM20470).
• Specialized military hardware transferred to SAE Aerospace Standards (AS), maintaining form, fit, and mechanical equivalence under commercial administration.

Structural Interchangeability Rules

Upgrading from an AN or MS fastener to an equivalent NAS or SAE AS component is generally structurally allowable if head clearances permit. Downgrading from an NAS or AS close-tolerance bolt to a standard AN or MS bolt is strictly forbidden unless explicitly approved by certified engineering data or Supplemental Type Certificate (STC) documentation.

Quality Assurance, Traceability & Compliance

Flight-critical hardware mandates verified supply chain documentation:

• AS9100D & NADCAP Accreditation: Manufacturers must operate under AS9100D quality management systems, utilizing NADCAP-accredited processing facilities for heat treating, plating, and testing.
• Certificate of Conformance (CoC): Official documentation verifying hardware compliance with governing drawings and specification revisions.
• Material Test Reports (MTRs): Chemical, physical, and thermal test data directly traceably linked to the metal's heat lot number.

Why Procurement Teams Trust ASAP Components 

Procurement specialists and aerospace buyers rely on ASAP Components for full supply chain authenticity, material traceability, and adherence to AS9100D and ISO quality standards. Access to extensive stock covering obsolete, hard-to-find, and standard AN, MS, NAS, and SAE AS hardware enables purchasing teams to reduce lead-time bottlenecks while staying fully compliant with defense and FAA requirements. 

Conclusion

Correct aerospace fastener selection is vital to maintaining airframe safety and structural integrity. Understanding the distinct differences across AN, MS, NAS, and AS standards prevents hazardous part substitutions. Always cross-reference part numbers against approved engineering drawings, OEM manuals, and official STC documents prior to finalizing hardware callouts.

Frequently Asked Questions

What are the key differences between MS and NAS bolts?

MS bolts are standard-tolerance fasteners for non-critical applications, while NAS bolts feature high tensile strength (160,000+ psi) and close-tolerance shanks built for flight-critical structural joints.

What are Aerospace Standards (AS) and who governs them?

Aerospace Standards (AS) are technical hardware and material specifications governed by SAE International that serve as modernized commercial replacements for legacy military specs.

What are the primary types of structural aerospace fasteners?

Aerospace fasteners primarily consist of structural bolts, solid/blind rivets, lockbolts/pin fasteners (like Hi-Lok), and self-locking nuts or washers.

What is the most common aviation fastener used industry-wide?

The AN3 through AN8 bolt series remains most common in light civil aviation, whereas modern commercial production relies heavily on close-tolerance NAS6600 series bolts and Hi-Lok pin systems.