AN6-56A – AN Standard Hex Head Airframe Bolt (Undrilled Shank) 🔩
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Overview
The AN6-56A is a standardized, Hex Head Airframe Bolt manufactured to the rigorous specifications of the AN (Air Force-Navy) Standard series of aerospace fasteners. This bolt is designated for use in structural joints where high shear strength and tensile strength are mandatory for airworthiness. The part number breakdown is technical: AN (Standard), 6 (nominal diameter of 6/16 or 3/8 inch), 56 (grip length in 1/16 inch increments, 56/16 or 3 1/2 inches), and the suffix A (signifying an undrilled shank and undrilled head).
The primary engineering purpose of the AN6-56A is to maintain a secure and reliable connection between structural members. It is specifically designed so that the unthreaded shank—the accurately machined, fine-tolerance portion—bears the shear loads of the joint. This prevents the stress-concentrating threads from being positioned within the material stack-up, which is a critical design requirement for maximizing the fatigue life of the entire structure. As an AN standard bolt, it guarantees the metallurgical properties, dimensional accuracy, and strength ratings necessary for flight-critical applications.
Key Features
Undrilled (Plain) Shank: The 'A' suffix designates the absence of a cross-drill hole in the shank, meaning this bolt is intended to be secured with a prevailing torque or all-metal self-locking nut (e.g., MS21042 or MS20365), and not a castle nut and cotter pin.
High-Strength Alloy Steel: Forged and heat-treated from low-carbon alloy steel (commonly 4037, 8740, or 4130) to meet the mandated high shear and minimum 125,000 PSI tensile strength requirements.
Extended Grip Length: The 3 1/2 inch grip length is designed to accommodate thick structural members or a large stack of sheet metal, ensuring the entire shear plane is carried by the precision-ground shank.
Cadmium Plated Finish: Protected by a Cadmium plating (QQ-P-416, Type II, Class 3) for exceptional resistance to atmospheric and galvanic corrosion, a standard finish for aerospace steel fasteners.
UNF Fine Thread: Features the Unified Fine (UNF) thread form, which offers a larger stress area and greater resistance to thread stripping and loosening from vibration compared to coarse threads.
Technical Specifications
Part Number: AN6-56A
Component Type: Hex Head Airframe Bolt (Undrilled)
Governing Specification: MIL-B-6812 (superseded by NASM6812)
Nominal Diameter (AN-Code 6): 3/8 inch (0.375 in.)
Total Length (Nominal): 4 1/16 inches (4.0625 in.)
Grip Length (AN-Code 56): 56/16 inches or 3 1/2 inches (3.500 in.) (±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)
Minimum Tensile Strength: 125,000 PSI (Pounds per Square Inch)
Finish: Cadmium Plated (AMS-QQ-P-416, Type II, Class 2 or 3)
Aircraft Compatibility
Primary Usage: A fundamental structural fastener found across all sectors of the aerospace industry, including:
General Aviation (GA) Aircraft (e.g., control surface hinges, large sheet metal stacks)
Rotorcraft (Non-rotating structures)
Commercial and Military Aircraft (Secondary and primary structural fittings)
Application Areas: Utilized in high-load tension and shear joints where the 3 1/2 inch material stack-up dictates the necessary grip length, such as in spar assemblies or heavy rib-to-skin attachments.
Verification Note: The Structural Repair Manual (SRM), Illustrated Parts Catalog (IPC), or Engineering Drawings for the specific repair or assembly are the absolute final authority for the correct part selection.
Applications Section
The AN6-56A is crucial for the integrity of airframe assemblies, serving in:
Heavy Fittings: Securing thick brackets or heavy fittings to bulkheads or spars where the long grip length is essential.
Stacked Laminates: Joining assemblies comprised of multiple layers of metal or composite material, ensuring the threads do not interfere with the bearing surfaces.
Control Cable Pulleys/Bellcranks: Attaching critical components that transmit pilot input forces, requiring a reliable, non-vibration-prone fastener.
Why Choose the AN6-56A AN Standard Bolt?
The decision to use a certified AN standard bolt like the AN6-56A is a requirement for maintaining the aircraft's Type Certificate (TC) basis. This bolt's 125,000 PSI minimum tensile strength is a verified characteristic, achieved through strict metallurgical control and heat-treatment procedures that ensure fracture toughness and predictable performance under extreme dynamic loading. Substituting this fastener with a commercial bolt, even one with a seemingly similar physical size, risks installing hardware with unknown strength, insufficient fatigue resistance, or an improper Grip/Thread runout ratio. In a shear joint, using an improperly sized or weaker bolt can lead to bearing failure of the material or premature fracture of the fastener itself, directly compromising flight safety.
The proper selection and use of the AN6-56A are governed by the Grip Length rule, which directly addresses the physics of shear joint mechanics. In a highly stressed joint, the bolt is subject to forces that attempt to slice it across its axis. If the threads were in the shear plane, the sharp root of the thread acts as a stress concentration factor (Kt major), drastically lowering the cross-sectional area and creating an initiation point for fatigue cracks. The long, smooth shank of the AN6-56A provides the maximum possible shear area and distributes the load uniformly across the bearing surface of the joined members. Furthermore, achieving the correct preload (tension) by torquing the nut compresses the joint, creating a high-friction clamp that resists relative movement (slip) between the parts, which is the mechanism that prevents fretting corrosion and minimizes fatigue loading on the bolt itself.
Note: The AN6-56A is a flight-critical structural fastener. The long grip length of 3 1/2 inches mandates extreme care in ensuring the grip length exactly matches the material stack-up of the joint. Inaccurate grip length selection can lead to thread engagement in the shear plane, compromising the joint's integrity. The bolt must be secured with an appropriate self-locking nut torqued to the specific value prescribed in the maintenance manual for the 3/8 inch, 125,000 PSI minimum tensile strength bolt. The bolt must not be reused if it shows any sign of distortion, damage, or plating loss. 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 and torque values.