AN4-12 – 1/4-28 x 1-1/4" Drilled Shank Hex Bolt, Alloy Steel, Cadmium Plated
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Overview
The AN4-12 is a high-strength, general-purpose aerospace fastener manufactured to the enduring Air Force-Navy (AN) standard. This hex head bolt is precision-machined from heat-treated alloy steel and is designed for critical structural joints, particularly those loaded in shear. Its defining characteristic is a shank drilled with a transverse hole for the installation of a cotter pin. This feature designates its use with a castellated nut (such as an AN310) to create a positive locking mechanism that physically prevents the nut from rotating, ensuring the ultimate security of the joint.
With a 1/4-inch diameter and a 1-1/4-inch nominal length, this cadmium-plated bolt is a staple in aircraft assembly and repair. Its adherence to the AN specification guarantees the material strength, dimensional accuracy, and traceability required for flight-critical applications, from flight control systems to landing gear linkages.
Key Features
High-Tensile Alloy Steel: Forged from premium alloy steel (typically 4140 or 8740) and heat-treated to a minimum tensile strength of 125,000 PSI for maximum joint strength.
Drilled Shank for Positive Locking: Features a hole in the threaded portion to accommodate a cotter pin, for use with a castellated nut. This provides a secure, non-preload-dependent locking system.
Optimized for Shear Loads: The substantial grip length and positive locking system make this bolt ideal for shear applications, such as in clevises and control system linkages.
Cadmium Plating: Coated with a sacrificial cadmium layer per QQ-P-416 for superior corrosion resistance and consistent performance.
Rolled Threads: Features 1/4-28 UNF-3A threads that are cold-rolled, a process that significantly strengthens the thread root and increases fatigue life compared to cut threads.
AN Standard Compliance: Guarantees every aspect of the bolt meets the rigorous quality and traceability standards required for airworthy components.
Technical Specifications
Part Number: AN4-12
Component Type: Drilled Shank Hex Head Bolt
Governing Specification: AN4 / NASM4
Material: Alloy Steel (e.g., 4140, 8740), heat-treated
Minimum Tensile Strength: 125,000 PSI
Finish: Cadmium Plate per QQ-P-416, Type II, Class 2
Diameter: 1/4 inch (0.250 inches)
Nominal Length (under head to end): 1-1/4 inches (1.250 inches)
Grip Length (Unthreaded Shank): 13/16 inch (0.813 inches)
Thread Specification: 1/4-28 UNF-3A
Aircraft Compatibility
The AN4-12 is a universal standard hardware item used across the entire aerospace industry. It is not specific to one aircraft but is a fundamental component found on:
Military Aircraft
Commercial and Transport Category Aircraft
General Aviation (Piston and Turbine)
Rotorcraft
Note: Verification of applicability is the responsibility of the installer. The grip length must be correctly matched to the total thickness of the material being joined. Always consult the current version of the aircraft's Illustrated Parts Catalog (IPC) or maintenance manual to confirm part number eligibility.
Applications
Flight Control Systems: Attaching rod ends, bellcranks, and control horns where a positive lock is mandatory.
Landing Gear Systems: Used in linkages, braces, and scissors assemblies.
Structural Joints: Any critical shear-loaded joint that requires a non-preload-dependent locking mechanism for ultimate safety and reliability.
Why Choose AN4-12?
For flight control systems and other critical joints subject to motion or vibration, a positive locking mechanism is paramount. The AN4-12, when paired with a castellated nut and cotter pin, provides a physically locked system that cannot loosen even if the joint's preload relaxes. Using a certified AN-spec fastener is the only way to guarantee the material strength, fatigue life, and dimensional precision required for these applications. The substitution of non-certified hardware represents a direct and unacceptable compromise of flight safety.
The engineering principle behind the AN4-12 bolt's drilled shank is the creation of a "positive lock." This is fundamentally different from the "prevailing torque" lock of a self-locking nut. A prevailing torque nut relies on friction to resist loosening. A positive lock, achieved by installing a cotter pin through the castellated nut and the bolt's shank, creates a physical obstruction. This system provides ultimate security in joints that may be subject to rotation or where clamp load is either not critical or could be lost in service. Even if the nut loses all its torque, the cotter pin physically prevents it from rotating more than a few degrees, ensuring the complete disengagement of the fastener is impossible. This is why this system has been the gold standard for decades in primary flight control and landing gear systems.
Note: This is a flight-critical hardware component. Installation must be performed by a certified mechanic. The nut should be torqued to the value specified in the maintenance manual, which is typically a low value intended to seat the joint without placing the bolt in high tension. After torquing, if the slots in the nut do not align with the hole in the bolt, the nut should be tightened slightly to the next available slot—never loosened. A new cotter pin must be used for every installation.
Documentation
The specifications for this fastener are maintained under the National Aerospace Standards (NASM), which have superseded many of the original AN drawings. Publicly accessible summaries of these standards can often be found through standards organizations and authorized hardware suppliers.