AN6H-22 – AN Standard Hex Head Airframe Bolt (Shank Drilled) 🔩
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Overview
The AN6H-22 is a high-strength, flight-critical Hex Head Airframe Bolt manufactured to the stringent requirements of the AN (Air Force-Navy) Standard aerospace hardware specification. This bolt is specifically designed for use in structural joints where the bolt is subject to high shear and tensile loads, and where a positive mechanical lock (safety wire or cotter pin) is required for the nut. The part number breakdown is precise: AN (Standard), 6 (nominal diameter of $6/16$ or $3/8$ inch), H (signifying the head is drilled for safety wiring), and 22 (grip length in $1/16$ inch increments, $22/16$ or $1 \text{ 3/8}$ inches).
The primary function of the AN6H-22 is to provide a reliable, predictable clamping force within airframe assemblies. It is manufactured from heat-treated alloy steel to ensure a minimum tensile strength of 1$125,000$ PSI.2 The bolt's shank-drilled feature is paramount, as it indicates the fastener is intended for use with a castle nut (e.g., AN310 or AN320) secured by a cotter pin or safety wire through the shank, providing a positive, failsafe lock against vibration-induced loosening. The unthreaded Grip Length is crucial, as it is dimensioned to carry the entire shear load across the joint members.
Key Features
Shank-Drilled for Positive Lock: The shank is drilled at its threaded end to accept a cotter pin when used with a castle nut, ensuring the joint cannot loosen under severe vibration—a mandatory feature for control linkages and critical rotating assemblies.
Head Drilled for Safety Wire: The 'H' in the part number indicates the head corners are drilled for safety wiring the bolt to prevent rotation, often used when multiple fasteners are grouped (e.g., engine mount bolts).
High-Strength Alloy Steel: Forged and heat-treated to achieve a reliable minimum tensile strength of 125,000 PSI, suitable for primary and secondary structural applications.3
Precision Grip Length: The $1 \text{ 3/8}$ inch grip is meticulously machined to ensure that the threads do not enter the shear plane of the joint, which is vital for maximizing fatigue life and preventing stress risers.
Cadmium Plated Finish: Protected by a Cadmium plating (QQ-P-416, Type II, Class 3) for exceptional resistance to atmospheric and galvanic corrosion, critical for long service life.4
Technical Specifications
Part Number: AN6H-22
Component Type: Hex Head Airframe Bolt (Shank and Head Drilled)5
Governing Specification: MIL-B-6812 (superseded by NASM6812)
Nominal Diameter (AN-Code 6): $3/8$ inch (0.375 in.)
Total Length (Nominal): $2 \text{ 11/16}$ inches (2.6875 in.)
Grip Length (AN-Code 22): $22/16$ inches or $1 \text{ 3/8}$ inches (1.375 in.) ($\pm 0.016$ in. tolerance)
Thread Length (Minimum): $0.609$ inches
Thread Specification: $3/8-24$ UNF-3A (Unified Fine thread, Class 3A fit)
Material: Alloy Steel (e.g., 4037, 8740, or 4130)6
Minimum Tensile Strength: 125,000 PSI (Pounds per Square Inch)7
Finish: Cadmium Plated (AMS-QQ-P-416, Type II, Class 2 or 3)8
Aircraft Compatibility
Primary Usage: A fundamental structural and control system fastener used across virtually all certified aircraft types, including:
General Aviation (GA) Aircraft (Cessna, Piper, Beechcraft)
Rotorcraft (Control linkages and transmission mounts)
Commercial and Military Aircraft (Structural fittings and access panels)
Application Areas: Mandated for use in any joint where the bolt is subject to rotation or where a specific preload must be maintained using the castle nut and cotter pin method. Typical applications include flight control hinges, engine controls, and landing gear attachments.
Verification Note: The Illustrated Parts Catalog (IPC) and Structural Repair Manual (SRM) are the definitive and final authority for the correct part selection, particularly the crucial grip length.
Applications Section
The AN6H-22 is deployed in safety-critical assemblies, serving:
Control Linkage Joints: Securing bellcranks, clevises, and push-pull rods where cotter pin safety is mandatory.
Engine Accessories: Mounting components where the castle nut and cotter pin system is used to prevent the fastener from backing off due to engine vibration.
Non-Sliding Shear Joints: Creating high-strength connections in structural members where the $1 \text{ 3/8}$ inch grip is appropriate.
Why Choose the AN6H-22 AN Standard Bolt?
The AN6H-22 is defined by its ability to accept a positive mechanical locking device via the drilled shank, a requirement for high-vibration and dynamically loaded joints where failure is catastrophic. The certified alloy steel and manufacturing process guarantee a minimum 125,000 PSI tensile strength and predictable ductility, ensuring the bolt will stretch before snapping under extreme force, providing a vital safety margin. Using a non-certified bolt, or an AN bolt with an incorrect grip length, compromises the essential principle of shear load bearing on the smooth shank and voids the certified locking method, directly risking component detachment and airframe integrity.
The UNF-3A thread form ($\frac{3}{8}$-24) is mandated not only for its tensile strength advantage but also for its precise interface with the mating nut, ensuring that the locking torque of the nut (whether castle or self-locking) is transmitted reliably. The addition of the drilled shank allows the joint to be torqued to the prescribed preload and then locked via the cotter pin or safety wire, fulfilling the concept of a redundant locking mechanism. The material's heat treatment controls its crystalline structure, ensuring a high yield strength (the point where plastic deformation begins) while maintaining a sufficient ultimate tensile strength, guaranteeing a high clamping force without risk of sudden, brittle fracture under impact.
Note: The AN6H-22 is a flight-critical structural fastener. The bolt must be discarded and replaced if the drilled holes show any signs of elongation or deformation. When securing the castle nut with a cotter pin, ensure the nut is only tightened to the next available castellation for pin alignment; never loosen the nut to align the pin hole. The Grip Length ($1 \text{ 3/8}$ in.) must precisely match the material stack-up of the joint. Installation and torque application must be performed and certified by a licensed A&P mechanic in accordance with AC 43.13-1B and the aircraft's maintenance manuals.
Documentation
Authoritative documentation for this part is held within the military standard:
NASM6812 (Bolt, Hex Head, Aircraft): The governing specification for the physical and metallurgical properties of AN bolts.
FAA Advisory Circular AC 43.13-1B, Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair: Contains tables and instructions for proper bolt selection, torque values, and safetying procedures.