AN Bolts: Aviation Standards, Markings, and Sizing
AN bolts (Army-Navy aeronautical standard fasteners) represent the gold standard in precision structural hardware utilized across commercial aviation, aerospace defense, and professional motorsports engineering. Developed during World War II to standardize military manufacturing parts across Allied aircraft branches, AN fasteners are manufactured to uncompromising dimensional tolerances and material metallurgical standards. Understanding AN head markings, grip length calculations, and shear ratings ensures flight-critical structural assemblies remain failure-proof under extreme cyclical vibrations.
Metallurgy, Cadmium Plating, and Head Markings
Standard commercial automotive grade-5 or grade-8 bolts are strictly prohibited in aircraft airframes due to unpredictable tensile strength, brittle failure modes, and loose thread tolerances. Standard AN structural bolts (series AN3 through AN20) are manufactured from high-tensile cadmium-plated alloy steel (typically AISI 4037 or 8740 alloy steel) or corrosion-resistant stainless steel. They deliver a minimum tensile strength of 125,000 PSI and a minimum shear strength of 75,000 PSI.
Deciphering an AN bolt begins with inspecting the markings stamped into the hex head. Standard cadmium-plated alloy steel AN bolts feature a raised asterisk or cross (+), certifying compliance with military specification MIL-B-6812. Corrosion-resistant stainless steel AN bolts feature an indented dash (-), while specialized close-tolerance bolts feature an embossed triangle containing an X.
Review this reference guide detailing common AN bolt head markings, material alloys, and physical strength specifications.
| Head Marking Symbol | Material Metallurgical Alloy | Tensile Strength Rating | Aviation Application Role |
|---|---|---|---|
| Raised Asterisk / Cross (+) | Cadmium-plated alloy steel (AISI 8740) | 125,000 PSI minimum tensile | Standard airframe structural joints & wing spars |
| Indented Dash (-) | Corrosion-resistant 300 series stainless | 80,000 to 125,000 PSI | Corrosive marine environments & battery bays |
| Two Raised Dashes (--) | High-strength 2024-T4 aluminum alloy | 62,000 PSI tensile | Lightweight non-structural cabin brackets |
| Raised Triangle with X | Close-tolerance ground alloy steel | 125,000 PSI (micrometer fit) | Flight control hinges & high-shear pin pivots |
| Recessed Dot (NAS spec) | Low-magnetic alloy steel | 160,000+ PSI extreme shear | Engine mounting cradles & landing gear trunnions |
Cadmium electroplating imparts a characteristic golden-yellow sheen that acts as a sacrificial corrosion barrier while providing natural thread lubrication.
Part Number Decoding and Grip Length Engineering
Deciphering an AN part number follows a strict alphanumeric system. In a part number like AN4-14A, the first digit following AN denotes the shank diameter in sixteenths of an inch (AN4 = 4/16 or 1/4-inch diameter). The number following the dash denotes the bolt length in eighths of an inch, with the final digit representing additional sixteenths (AN4-14 = 1-4/8 or 1.5 inches long). The trailing letter A signifies an un-drilled bolt shank; the absence of an A indicates a cotter-pin hole drilled through the threads.
A golden rule of aviation mechanical design is that threads must never bear in shear. The smooth unthreaded shank (grip length) must extend completely through all structural material being joined. The threaded portion exists solely to accommodate the washer and aircraft self-locking nut. Running threads inside a shear joint creates severe stress risers that lead to catastrophic metal fatigue fractures under flight turbulence.
The following table details AN bolt diameter sizing, thread pitches, and minimum single shear load ratings across standard airframe hardware.
| AN Series Designation | Nominal Bolt Diameter | Thread Pitch (UNF) | Minimum Single Shear Strength |
|---|---|---|---|
| AN3 | 3/16" (0.1875 in) | 10-32 UNF | 2,125 lbs single shear load |
| AN4 | 1/4" (0.2500 in) | 1/4-28 UNF | 3,680 lbs single shear load |
| AN5 | 5/16" (0.3125 in) | 5/16-24 UNF | 5,750 lbs single shear load |
| AN6 | 3/8" (0.3750 in) | 3/8-24 UNF | 8,280 lbs single shear load |
| AN7 | 7/16" (0.4375 in) | 7/16-20 UNF | 11,250 lbs single shear load |
| AN8 | 1/2" (0.5000 in) | 1/2-20 UNF | 14,700 lbs single shear load |
Utilizing precision AN960 flat washers allows mechanics to fine-tune grip length so locking nuts tighten without bottoming on unthreaded shanks.
How to Measure and Install an AN Bolt Joint in 5 Steps
Follow this FAA-standard airframe mechanical assembly procedure to select grip length, install washers, and torque AN bolts safely.
Measure Material Grip Thickness
Measure total combined thickness of all materials to be clamped using a vernier caliper or aviation grip gauge.
Select AN Bolt With Matching Grip Length
Choose an AN bolt whose unthreaded shank length equals or slightly exceeds the material thickness so threads remain outside the hole.
Install AN960 Aircraft Washers
Place an AN960 flat washer under the bolt head and beneath the nut to distribute clamping pressure and adjust thread clearance.
Orient Bolt Head Forward or Upward
Insert the bolt so the head faces forward, upward, or inboard, ensuring gravity or airflow prevents the bolt from dropping out if the nut fails.
Torque to FAA Advisory Circular 43.13 Specs
Torque the aircraft self-locking nut using a calibrated torque wrench, verifying 1.5 to 3 threads project beyond the nut face.
Frequently Asked Questions (8 Questions Answered)
Q1: What does AN stand for in AN bolts?
AN stands for Army-Navy, representing unified military standard hardware specifications established for aircraft manufacturing during World War II.
Q2: Why can't I use Grade 8 hardware store bolts on an airplane?
Grade 8 commercial bolts lack certified shear testing, have inconsistent grip lengths that place threads in shear, and lack aviation anti-corrosion plating.
Q3: What does the letter A at the end of an AN part number mean?
The letter A indicates an un-drilled shank (e.g., AN4-7A); omitting the A means the bolt has a drilled cotter-pin hole through the threaded end.
Q4: What is the rule for bolt head orientation on aircraft?
Aviation standard practice dictates that bolt heads should point forward, upward, or outboard so aerodynamic drag or gravity keeps the bolt in place if the nut is lost.
Q5: What is the difference between AN and NAS bolts?
AN bolts are standard 125,000 PSI fasteners, while NAS (National Aerospace Standard) bolts are higher-tensile (160,000+ PSI) fasteners used in extreme load applications.
Q6: How many threads should extend past an aircraft locknut?
According to FAA AC 43.13, a minimum of one to three full threads must project through the fiber or all-metal locking element of the nut.
Q7: What does the cross stamped on an AN bolt head mean?
A raised cross or asterisk certifies that the bolt is manufactured from certified cadmium-plated nickel-chromium alloy steel (125,000 PSI tensile).
Q8: What washers are used with AN bolts?
AN960 flat washers (regular thickness) and AN960L (light/thin thickness) are used to adjust bolt grip lengths and prevent surface galling.
Final Thoughts & Key Takeaways
In conclusion, understanding an bolts: aviation standards, markings, and sizing provides essential clarity, practical strategies, and actionable advice. By incorporating these foundational insights, adhering to verified safety guidelines, and following structured best practices, you ensure reliable, long-term outcomes while preventing common mistakes. Stay informed, consult certified professionals when needed, and maintain consistent quality care.