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
Cross or asterisk
AN standard steel bolt
Alloy steel (cadmium-plated)
125,000 psi
The general-purpose aircraft bolt
Single raised dash
AN corrosion-resistant steel bolt
Corrosion-resistant steel (CRES)
125,000 psi
Used where corrosion resistance is required
Two raised dashes
AN aluminum alloy bolt
2024-T4 aluminum alloy
62,000 psi
Much lower strength — use only where specified; not for tension loads or frequent removal
Triangle
Close-tolerance bolt (e.g. AN173–AN186, NAS)
Steel or aluminum alloy
Per specification
Ground shank for a tight fit — used where a drive fit is required
Raised or recessed "R" / "SPEC"
Special or manufacturer bolt
Varies
Varies
Use only as specified in the aircraft maintenance manual or approved data
Use with the matching high-strength nut and washers — do not substitute
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 gives the length: the last digit is eighths of an inch and any digits before it are whole inches (-5 = 5/8", -12 = 1-1/4"). An "A" suffix means an undrilled shank; no suffix means the shank is drilled for a cotter pin. An "H" after the diameter number means a drilled head.
AN4 = 1/4" bolt, -10 = 1 inch and 0/8 = 1" long, A = undrilled
Digits before the last are whole inches
AN4-14A
AN4 = 1/4" bolt, -14 = 1 inch and 4/8 = 1-1/2" long, A = undrilled
AN4H5A
H = drilled head (for safety wire), -5 = 5/8" long, A = undrilled shank
Clevis bolts are a separate series (AN21–AN37)
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 — needs a lock washer, check nut or safety wire
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 and the rivet shank swells or buckles in the hole and the joint is weakened; too tight and the rivet won't seat properly. 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. Use stainless steel or Monel for safety wiring. Soft copper wire is used only for shear and seal wiring (such as emergency handles and seals), where it is meant to break.
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 pull direction
Install so the wire pulls each fastener in its tightening direction (clockwise for right-hand threads) — the wire must never be able to let a fastener back off
Twist rate
6–8 twists per inch for 0.032" and 0.041" wire; 8–14 for 0.020" wire — 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 a castle slot and the drilled hole in the bolt shank
Double-twist method
Two strands twisted together — the standard method for most fasteners
Single-wire method
One strand — for small screws in a closely spaced closed pattern, electrical systems, and hard-to-reach places
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