AN8-15A – AN Standard Hex Head Airframe Bolt (Undrilled Shank) 🔩
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Overview
The AN8-15A is a fundamental component of the AN (Air Force-Navy) Standard series of aerospace hardware, representing a flight-critical Hex Head Airframe Bolt.1 It is a standardized tension and shear fastener designed for general purpose structural use in aircraft construction, maintenance, and repair, manufactured strictly to the requirements of the governing military specification (MIL-B-6812). The code breakdown signifies: AN (Air Force-Navy standard), 8 (nominal diameter of $8/16$ or $1/2$ inch), 15 (grip length in $1/16$ inch increments, $15/16$ inch), and the suffix A (signifying an undrilled shank and undrilled head).
This bolt's primary function is to create a secure, high-strength bolted joint capable of withstanding the complex static, dynamic, and fatigue loads inherent in an airframe. Unlike commercial fasteners, the AN8-15A is manufactured from heat-treated alloy steel to achieve a precisely controlled minimum tensile and shear strength, and features a fine-tolerance shank known as the Grip Length.2 The length and diameter are precisely defined to ensure the unthreaded portion (shank) carries the shear load across the joint members, preventing thread runout in the bearing surface—a critical requirement for maintaining joint fatigue life.
Key Features
Undrilled (Plain) Shank: Designated by the 'A' suffix, the shank is undrilled, indicating this bolt is to be secured with a positive-locking nut (e.g., MS21042 or AN365) and not a castle nut secured by a cotter pin through the shank.
High-Strength Alloy Steel: Manufactured from specialized, heat-treated alloy steel (typically 4037, 8740, or 4130) to achieve a high-performance balance of strength and ductility.
Fine-Tolerance Grip Length: The unthreaded portion (grip) is precisely ground to a tight tolerance to ensure a near-perfect fit in the hole, minimizing joint movement and resisting fretting corrosion and wear under shear loading.
Cadmium Plated Finish: Protected by a Cadmium plating (QQ-P-416, Type II, Class 3) for exceptional resistance to atmospheric and galvanic corrosion.
Hex Head Design: Facilitates precise torque application for achieving the required preload (clamp-up) in the bolted joint.
Technical Specifications
Part Number: AN8-15A
Component Type: Hex Head Airframe Bolt
Governing Specification: MIL-B-6812 (superseded by NASM6812)
Nominal Diameter (AN-Code 8): $1/2$ inch ($0.500$ in.)
Total Length (Nominal): 8$1 \frac{23}{32}$ inches (9$1.719$ in.)
Grip Length (AN-Code 15): 11$\frac{15}{16}$ inch (12$0.938$ in.) (13$\pm 0.016$ in.14 toleranc
Thread Length (Minimum): 15$0.781$ inches
Thread Specification: 17$\frac{1}{2}-20$ UNF-3A (Unified Fine thread, Class 3A fit)
Material: Alloy Steel (e.g., 4037, 8740, or 4130)
Heat Treatment/Hardness: Hardness Rating HRC 26–3220
Minimum Tensile Strength: 21$125,000$ PSI (Pounds per Square Inch)
Finish: Cadmium Plated (AMS-QQ-P-416, Type II, Class 2 or 3)
Aircraft Compatibility
Primary Usage: A universal standard bolt used across virtually all classes of aircraft, including:
General Aviation (GA) Aircraft (Cessna, Piper, Beechcraft)
Commuter/Regional Aircraft
Legacy Military Aircraft
Application Areas: Used in areas requiring high-strength fasteners, such as: engine mounts, flight control hinges, landing gear attachments, wing spars, and bulkhead connections, provided the required length and grip are verified.
Verification Note: The Illustrated Parts Catalog (IPC), Structural Repair Manual (SRM), or relevant Engineering Drawings for the specific application are the definitive and final authority for selecting the correct AN bolt part number.
Applications Section
The AN8-15A is primarily used in high-shear and moderate-tension structural joints. Its applications include:
Engine Mount Connections: Where the bolt secures engine mounting bushings to the firewall structure.
Control Surface Attachment: Bolting clevises, brackets, or hinges for control systems where the undrilled shank is preferred for structural reasons.
Airframe Assembly: General-purpose fastening of airframe members and fittings where a tight-fitting, high-strength shear fastener is mandatory.
Why Choose the AN8-15A AN Standard Bolt?
The distinction between a certified AN standard airframe bolt and a commercial-grade fastener is a matter of life-safety metallurgy and precision engineering. The AN8-15A is manufactured to a minimum tensile strength of 24$\mathbf{125,000}$ PSI, a rating substantially higher and more reliable than most commercial Grade 5 or even Grade 8 bolts.25 Crucially, the material is heat-treated to ensure a desirable level of ductility, meaning it will yield (bend) before catastrophic failure (snapping) under extreme overload—a vital failsafe in dynamic airframe structures. The standardized grip length ensures that the full shear load is carried by the unthreaded shank, maintaining joint integrity and significantly increasing the joint's fatigue life, thereby eliminating the risks associated with substituting with non-certified, poorly controlled hardware.
The proper function of the AN8-15A bolt as a shear fastener is rooted in the stress concentration factor ($\text{K}_t$) at the thread root. The design dictates that the entire shear plane of the joint must pass through the precision-ground Grip Length portion of the bolt's shank. Threads introduce significant stress risers, and locating threads within the shear plane dramatically reduces the joint's strength and fatigue resistance. Furthermore, the 33$\frac{1}{2}-20$ UNF-3A thread specification is critical.34 The UNF (Unified Fine) series thread has a smaller pitch and larger minor diameter than coarse thread, increasing the stress area of the threaded section, thereby maximizing the bolt's resistance to tensile failure under preload. The Class 3A fit guarantees minimal clearance between the bolt and the mating nut, ensuring a robust, vibration-resistant connection.
Note: The AN8-15A is a flight-critical structural fastener. Proper sizing requires that the Grip Length (35$0.938$ in.36 nominal) must match the total thickness of the joined material stack-up to ensure that only the unthreaded shank is within the shear plane.37 This bolt must be secured with an appropriate self-locking nut (e.g., AN365) torqued to the specific value prescribed in the maintenance manual for the $1/2$ inch, $125,000$ PSI minimum tensile strength bolt. The bolt must not be reused if it shows any sign of distortion, elongation, thread damage, or cadmium plating wear. 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.38
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.