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Aviation Hardware
AN/MS bolt head markings, fastener identification, nut types, rivet identification and sizing, safety wire specifications, and cotter pin reference for aircraft maintenance.
AN/MS Bolt Head Markings
Bolt head markings identify the bolt specification, material, and strength. Always verify the correct bolt specification before installation — substituting bolts is not permitted without engineering approval.
| Head Marking | Spec | Material | Tensile | Notes |
|---|
| No marking | AN bolt — unclassified | Cadmium-plated steel | 125,000 psi | Most common aircraft bolt — general purpose |
| Single raised dash | AN bolt — corrosion resistant | Corrosion resistant steel (CRES) | 125,000 psi | Used where corrosion resistance required |
| Two raised dashes | Close tolerance AN bolt | Steel | 125,000 psi | Precision fit — used in close tolerance holes |
| Triangle | MS bolt | Alloy steel | 150,000 psi | Higher strength than AN bolts |
| Triangle + letter | MS close tolerance bolt | Alloy steel | 150,000 psi | Close tolerance, high strength |
| R or SPEC | Manufacturer specification | Varies | Varies | Consult aircraft maintenance manual for specific bolt |
| Star or asterisk | Drilled shank bolt | Steel | 125,000 psi | Drilled for cotter pin safety |
| Cross or X | Internal wrenching bolt | Alloy steel | 180,000 psi | 12-point internal drive — NAS bolt |
AN Bolt Part Number Reference
AN bolt part numbers encode diameter, length, and shank type. The number after "AN" indicates the diameter in 16ths of an inch.
| Part Number | Diameter | Thread Pitch | Notes |
|---|
| AN3 | 3/16" | 10-32 | Smallest common aircraft bolt — avionics brackets |
| AN4 | 1/4" | 28 | Standard airframe bolt — very common |
| AN5 | 5/16" | 24 | Medium structural bolt |
| AN6 | 3/8" | 24 | Larger structural bolt |
| AN7 | 7/16" | 20 | Heavy structural |
| AN8 | 1/2" | 20 | Heavy structural |
| AN10 | 5/8" | 18 | Engine mount and heavy structural |
| AN12 | 3/4" | 16 | Heavy structural — engine mounts |
AN Bolt Dash Number Decoding
The dash number encodes bolt length in 8ths of an inch. The "A" suffix indicates an undrilled shank. No suffix means the shank is drilled for cotter pin or safety wire.
| Example | Breakdown | Notes |
|---|
| AN4-5A | AN4 = 1/4" bolt, 5 = length in 8ths (5/8"), A = undrilled shank | No A suffix = drilled shank for cotter pin |
| AN4-5 | AN4 = 1/4" bolt, 5 = length in 8ths (5/8"), no suffix = drilled shank | Drilled for safety wiring or cotter pin |
| AN4-10A | AN4 = 1/4" bolt, 10 = length in 8ths (10/8" = 1-1/4"), A = undrilled | Longer bolt — undrilled |
| AN4H5A | H = reduced hex head (clevis bolt) | H designation = clevis/reduced head |
Aircraft Nut Types
Self-locking nuts with fiber/nylon inserts must never be used above 250°F — use all-metal self-locking nuts in high temperature areas.
| Part Number | Type | Material | Use |
|---|
| AN310 | Castle nut | Cadmium-plated steel | With cotter pin safety — control systems |
| AN315 | Plain hex nut | Steel | General purpose — requires safety wiring |
| AN316 | Check nut (jam nut) | Steel | Locks adjustment of turnbuckles and rod ends |
| AN320 | Shear castle nut | Steel | Low torque applications — with cotter pin |
| AN350 | Wing nut | Steel | Frequently adjusted connections |
| MS21042 | Self-locking nut (all-metal) | Alloy steel | High temperature — engine and exhaust areas |
| MS21083 | Self-locking nut (fiber insert) | Steel + nylon | General purpose — not above 250°F |
| AN365 | Self-locking elastic stop nut | Steel + fiber | Standard self-locking — general airframe |
Rivet Head Types
| Designation | Head Type | Use | Notes |
|---|
| AN426 | Countersunk (flush) 100° | Flush skin surfaces — aerodynamic applications | Most common in aircraft skin |
| AN470 | Universal head | General purpose — interior structure | Replaces both round and brazier head |
| AN441 | Flat head | Sheet metal — interior structural | Low profile |
| AN455 | Brazier head | Sheet metal — older aircraft | Largely replaced by AN470 |
| AN456 | Modified brazier | Sheet metal | Slightly lower profile than brazier |
| MS20470 | Universal head | Replacement for AN470 — current standard | Same as AN470 — current MS designation |
| MS20426 | Countersunk 100° | Flush applications — current standard | Replacement for AN426 |
Rivet Material Codes
The material code appears between the head designation and dash number in the part number (e.g. AN470AD4-6 — AD = 2117-T4 aluminum). AD rivets are the most common in general aviation.
| Code | Material | Shear Strength | Use |
|---|
| A | 1100 aluminum | 9,500 psi | Non-structural — fairings, fillers |
| AD | 2117-T4 aluminum | 26,000 psi | General purpose — most common aircraft rivet |
| D | 2017-T4 aluminum | 34,000 psi | Thicker structural members — must be refrigerated before driving |
| DD | 2024-T4 aluminum | 41,000 psi | High strength structural — refrigerate before use |
| B | 5056 aluminum | 28,000 psi | Magnesium alloy structure |
| E | 7050 aluminum | Not standard | High strength applications |
Rivet Drill Size Reference
Rivet holes must be drilled to the correct size — too loose causes rivet to fail in tension, too tight prevents proper bucking. Hole diameter should be 0.002\"–0.004\" larger than rivet diameter.
| Rivet Diameter | Drill Size | Hole Clearance | Notes |
|---|
| 3/32" (#40) | #40 (0.098") | +0.002" to +0.004" | Smallest common structural rivet |
| 1/8" (#30) | #30 (0.1285") | +0.002" to +0.004" | Most common size — general structure |
| 5/32" (#21) | #21 (0.159") | +0.002" to +0.004" | Medium structural |
| 3/16" (#10) | #10 (0.1935") | +0.002" to +0.004" | Heavy structural |
| 1/4" | F (0.257") | +0.002" to +0.004" | Large structural rivet |
Safety Wire Gauges
0.032\" stainless steel is the most common safety wire for general aircraft maintenance. Always use stainless steel or Monel — never use copper or aluminum safety wire on aircraft.
| Gauge | Diameter | Material | Use |
|---|
| 0.020" | 0.020 in / 0.508 mm | Stainless steel or Monel | Small fasteners, instruments, electrical connectors |
| 0.032" | 0.032 in / 0.813 mm | Stainless steel or Monel | Standard — most common size for AN3–AN6 bolts |
| 0.041" | 0.041 in / 1.041 mm | Stainless steel or Monel | Large fasteners — AN8 and above |
| 0.051" | 0.051 in / 1.295 mm | Stainless steel | Heavy duty — engine bolts, high-load fasteners |
| 0.060" | 0.060 in / 1.524 mm | Stainless steel | Very heavy applications |
Safety Wire Installation Rules
Per FAA AC 43.13-1B Chapter 7. Always refer to the applicable aircraft maintenance manual for specific requirements.
| Rule | Requirement |
|---|
| Wire twist direction | Twisted so that if bolt loosens, wire tightens — pull direction toward bolt head |
| Twist rate | 6–8 twists per inch — uniform and neat |
| Maximum span | 24 inches maximum between anchor points on any one piece of wire |
| Pigtail at end | Minimum 3 twists at cut end — fold back against structure to prevent snagging |
| Drilled head bolts | Safety wire passes through hole in bolt head — do not wrap around head |
| Castellated nuts | Wire passes through castle slots AND cotter pin hole in bolt shank |
| Double wrap method | Two wires twisted together — required on most structural fasteners |
| Single wrap method | One wire only — permitted on small fasteners in confined spaces |
Cotter Pin Quick Reference
| Part Number | Diameter | Length | Use |
|---|
| AN380-2-2 | 1/16" | 1/2" | AN3 castle nut |
| AN380-2-4 | 1/16" | 1" | AN3 longer bolts |
| AN380-3-5 | 3/32" | 1-1/4" | AN4 castle nut |
| AN380-4-4 | 1/8" | 1" | AN5 and AN6 castle nut |
| AN380-4-6 | 1/8" | 1-1/2" | AN6 and AN7 castle nut |
| AN380-5-6 | 5/32" | 1-1/2" | AN7 and AN8 castle nut |
Cotter Pin Installation Rules
| Rule | Requirement |
|---|
| Fit | Cotter pin must fit snugly in hole — do not use undersized pin |
| Prong direction (preferred) | Bend one prong over bolt end, one prong down along nut — preferred method |
| Prong direction (optional) | Both prongs bent over bolt end — acceptable if clearance allows |
| Never reuse | Cotter pins are single-use only — always install a new pin |
| Castle nut alignment | Torque nut to low end of spec first, advance to align slot — never back off |