AN4H3A - Drilled Head Cadmium Machine Bolt

AN4H3A – 1/4-28 x 3/8" Drilled Head Hex Bolt, Alloy Steel, Cadmium Plated

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Overview

The AN4H3A is a foundational piece of certified aerospace hardware, a high-strength, drilled head hex bolt manufactured to the stringent Air Force-Navy (AN) standard. This fastener is specifically engineered for critical tension-loaded joints where positive locking is mandatory. Forged from high-tensile alloy steel and protected by a durable cadmium plating, the AN4H3A is designed to resist loosening under the most severe vibration environments. Its defining feature is the drilled head, which allows for the application of lockwire, providing a physical, verifiable method of securing the fastener.

Every dimension, material property, and manufacturing process for this bolt is rigidly controlled by the AN specification, ensuring the traceability and uniformity required for flight-critical applications. From engine accessory mounts to structural brackets, the AN4H3A represents a legacy of proven reliability in aircraft construction and repair.

Key Features

• High-Tensile Alloy Steel: Manufactured from heat-treated alloy steel (typically 4140 or 8740) to a minimum tensile strength of 125,000 PSI, providing exceptional strength for tension applications.

• Drilled Head for Lockwiring: The hexagonal head is cross-drilled with multiple holes to facilitate the installation of safety wire (lockwire), creating a positive, anti-rotation locking mechanism.

• Cadmium Plating: Coated with a sacrificial cadmium layer per QQ-P-416 for superior corrosion resistance and consistent lubricity, ensuring accurate torque application.

• Rolled Threads: Features 1/4-28 UNF-3A threads that are cold-rolled, not cut. This process significantly increases fatigue life by improving the grain structure at the critical thread roots.

• AN Standard Compliance: As a true AN-standard part, it guarantees interchangeability, dimensional accuracy, and material integrity for airworthy installations.

• Tension Application Design: The hex head design is optimized for high clamp loads (preload) in joints where the primary force is axial, or in tension.

Technical Specifications

• Part Number: AN4H3A

• Component Type: Drilled Head Hexagonal Bolt

• Governing Specification: AN4 / NASM4 (or superseding standards)

• 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

• Nominal Diameter: 1/4 inch (0.250 inches)

• Nominal Length (under head to end): 3/8 inch (0.375 inches)

• Thread Specification: 1/4-28 UNF-3A

• Head Type: Hexagonal, drilled for safety wire

• Shank Condition: Undrilled (no hole for cotter pin)

Aircraft Compatibility

The AN4H3A is a standard hardware component, not specific to a single aircraft model. It is used ubiquitously across all segments of aviation where engineering drawings specify a 1/4-28 drilled head hex bolt of this length and material. Applications span:

• Military Aircraft

• Commercial and Transport Category Aircraft

• General Aviation (Piston and Turbine)

• Rotorcraft

Note: Verification of applicability is the responsibility of the installer. Always consult the current version of the applicable Illustrated Parts Catalog (IPC) or maintenance manual to confirm part number eligibility for a specific location.

Applications

• Engine accessory and component mounting (starters, alternators, magnetos)

• Firewall and engine baffle attachments

• Structural and secondary airframe brackets

• Hydraulic pump and component mounting

• Any critical bolted joint subject to high vibration where a positive lock is required and a castellated nut with a cotter pin is not used.

Why Choose AN4H3A?

In aviation, there is no substitute for certified hardware. An AN4H3A bolt guarantees the specific material chemistry, heat treatment, and manufacturing processes essential for flight safety. The use of a commercial-grade or non-traceable bolt would introduce a catastrophic risk of failure due to insufficient tensile strength, dramatically reduced fatigue life from cut threads, and unpredictable failure from corrosion. This certified fastener ensures that the joint will achieve and maintain the clamp load specified by the aircraft's design engineers.

The exceptional fatigue resistance of the AN4H3A bolt is a direct result of its manufacturing process, specifically the formation of its 1/4-28 UNF-3A threads. These threads are not created by a subtractive cutting process, but by cold rolling, where the bolt blank is forced between hardened dies. This method plastically deforms the alloy steel, causing the material's grain structure to flow smoothly and continuously along the contour of the thread profile. This process induces compressive residual stresses at the thread root, the most common site for fatigue crack initiation. By creating this compressive stress, the thread root is effectively strengthened against the cyclic tensile loads experienced in service, dramatically increasing the bolt's service life compared to a bolt with machine-cut threads, whose severed grain structure creates numerous stress risers.

Note: This is a flight-critical hardware component. Installation must be performed by a certified mechanic. A calibrated torque wrench must be used to apply the correct torque value specified in the maintenance manual to achieve proper preload. Safety wire must be installed in accordance with standard practices (e.g., AC 43.13-1B), ensuring it is routed and twisted to create a positive tightening force on the bolt head.

Documentation

While the original AN (Air Force-Navy) drawings are historical, the specifications for this type of fastener are maintained under the National Aerospace Standards (NASM). Publicly accessible summaries of these standards can often be found through standards organizations.

• Reference (Example Specification): MIL-DTL-18240F (Fastener, Externally Threaded, 250°F, Self-Locking) - Note: This is a related specification for self-locking fasteners, illustrating the type of public documentation available for aerospace hardware.