Walk into any aircraft builder's workshop and you'll find bolts marked AN, MS, and NAS sitting side by side in parts bins. They look nearly identical. They're not. Each specification carries different strength ratings, tolerances, and approval histories — and substituting one for another without verifying the design can have serious consequences. This guide covers every practical identifier you need to tell them apart on the workbench.
The Three Standards at a Glance
Before getting into head markings and measurement, it helps to understand where each standard came from and what it was designed to accomplish.
| Feature | AN (Air Force-Navy) | MS (Military Standard) | NAS (National Aerospace Standard) |
|---|---|---|---|
| Origin | WWII-era Air Force / Navy joint standard | Post-WWII DoD unification | Aerospace Industries Association |
| Status | Active; many superseded by MS | Active; supersedes many AN numbers | Active; maintained by AIA |
| Tensile Strength | ~125,000 psi (alloy steel) | Same as AN equivalent (direct replacements) | ~160,000 psi (close-tolerance series) |
| Tolerance | Standard clearance fit | Standard clearance fit | Close tolerance (±0.0005") |
| Primary Use | General structural, homebuilts, certified | Wherever plans specify MS part number | High-load, close-tolerance hole applications |
| Head Profile | Flat hexagonal, no dished face | Flat hexagonal (matches AN) | Cupped/dished hex face — most reliable visual ID |
AN Bolt Head Markings
AN bolts use a simple marking system stamped into the bolt head. Once you learn to read it, identification becomes quick even without a micrometer.
- Plain, unmarked head: Low-strength steel. Not suitable for primary structural applications.
- Two raised dashes (radial lines): Aluminum alloy 2024-T4. Lighter weight, lower strength — typically used only where weight savings are critical and loads permit.
- "C" suffix in part number (e.g., AN4C-7): CRES (Corrosion Resistant Steel) 300-series stainless. Identified by a single raised dash or "C" marking on head.
- "H" suffix (e.g., AN4H-7): Drilled head for safety wire. No change in material or strength versus undrilled equivalent.
- Raised triangle or no triangle: Standard AN alloy steel bolts (AN3 through AN20) are cadmium-plated alloy steel with approximately 125,000 psi minimum tensile strength. The cadmium plating gives them their characteristic silver-gray finish distinct from bare steel.
The most common AN bolts you'll handle — AN3 through AN8 — are all alloy steel cadmium-plated, and their head markings will be a plain flat hex face with a raised dash or no marking. The part number itself is the primary identifier.
MS Bolt Head Markings
MS replaced many AN part numbers after WWII as the DoD consolidated standards. Critically, many MS part numbers are direct form-fit-function equivalents of their AN counterparts — the part is identical, only the designation changed.
For example, AN365 castle nuts are also sold as MS20365 — same thread, same material, same geometry, same part. Similarly, AN3 bolts are cross-referenced to MS20033. When you see MS part numbers on plans or in FAA documentation, look up the AN cross-reference before assuming you need a different part.
Material markings on MS hardware follow the same convention as AN: two raised dashes for aluminum, "C" for CRES, plain head for standard alloy steel. The head profile remains a flat hexagonal face — the same visual footprint as AN hardware. This is why part number verification matters more than visual inspection for AN vs MS discrimination.
NAS Bolt Head Markings
NAS hardware has one reliable visual identifier that distinguishes it from AN and MS at a glance: the cupped or dished bolt head face. Where AN and MS bolts have a flat hexagonal top surface, NAS bolts have a slightly concave (dished inward) face. Look at the bolt head straight-on under good light — the concavity is visible as a subtle circular shadow.
- Cupped/dished hex face: The most reliable visual identifier — present on virtually all NAS close-tolerance bolts.
- Raised or recessed triangle: Secondary marking often found on NAS bolts, though not universally present across all NAS series.
- Tighter shank diameter: NAS close-tolerance bolts are ground to ±0.0005" versus AN/MS clearance tolerances. You cannot feel this difference by hand — it requires measurement.
- Higher strength: NAS close-tolerance bolts achieve approximately 160,000 psi minimum tensile strength, roughly 28% above standard AN alloy steel.
The tighter tolerance is the engineering reason NAS hardware exists. Close-tolerance holes loaded in shear transfer load through bearing contact rather than bolt bending — NAS hardware is machined to fill that hole precisely.
When Each Spec Is Required
The simplest rule: use what the plans specify. When you're working from approved drawings, the spec is not a judgment call. When you're building experimental and the plans give you flexibility, here's the decision logic:
- Use AN for general structural applications on homebuilts and production aircraft, standard-tolerance clearance holes, and any application where plans reference AN part numbers. The default choice for AN/MS hardware is AN unless otherwise specified.
- Use MS when plans or maintenance manuals specify MS part numbers. Since many MS bolts are direct AN equivalents, verify the cross-reference — you may already have the right part.
- Use NAS for close-tolerance hole applications where the fit is designed to be tight, higher-load primary structural joints where the extra strength margin is engineered in, and any location where drawings call out NAS hardware explicitly.
- Do not substitute NAS for AN in standard-tolerance holes without verifying the design. NAS bolts in oversized holes provide no benefit and can actually reduce the load path quality if hole preparation was done for clearance fit.
- Do not substitute AN for NAS in close-tolerance applications. The strength and fit margins that justify using NAS hardware are absent in AN bolts.
Material Differences
Across all three standards, the same material families appear. The head markings identify the material; the part number identifies the spec. Here is the full material reference:
| Material | Identifier | Typical Use |
|---|---|---|
| Alloy Steel (cadmium plated) | Plain head or no special marking; silver-gray finish | Primary structural bolts — most AN3–AN10 |
| CRES 300-series stainless | "C" suffix in part number; single raised dash on head | Corrosive environments, fuel systems, exhaust proximity |
| CRES A-286 (high-temp) | Specific NAS part numbers; silver finish, often magnetic | Turbine hot sections, high-temperature structural |
| Titanium | Specific NAS part numbers; distinctive dull silver color | Weight-critical primary structure, high-end composite builds |
| Aluminum 2024-T4 | Two raised dashes on bolt head | Non-critical, weight-sensitive applications only |
For the vast majority of homebuilt and MRO work involving AN/MS hardware, you'll be working with cadmium-plated alloy steel. CRES and titanium appear in specific design locations and are called out explicitly in drawings. See our AN3–AN10 Sizing Reference for diameter and thread specifications by bolt number, and our FAA 8130-3 guide for understanding traceability documentation. When comparing prices against other suppliers, our Aircraft Spruce comparison page shows current pricing on common AN bolt sizes.