AN4H4A - Cadmium Drilled Head Machine Bolt

AN4H4A – 1/4-28 x 1/2" Drilled Head Hex Bolt, Alloy Steel, Cadmium Plated

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Overview

The AN4H4A is a high-strength, drilled-head hex bolt manufactured to the rigorous Air Force-Navy (AN) standard. This fastener is specifically engineered for tension-loaded joints in high-vibration environments where a positive locking mechanism is mandatory. Its defining feature is the set of holes drilled through the flats of the hexagonal head, which are designed for the installation of safety wire (lockwire). With a 1/4-inch diameter and a short 1/2-inch nominal length, this bolt is a staple for securing components and brackets where clearance is limited and reliability is paramount.

Forged from heat-treated alloy steel and protected by a durable cadmium plating, the AN4H4A provides both exceptional strength and corrosion resistance. Its adherence to the AN specification ensures the full traceability and quality control required for any flight-certified application.

Key Features

• Drilled Head for Lockwiring: The hexagonal head is cross-drilled to facilitate the installation of safety wire, creating a positive, anti-rotation locking mechanism that is visually verifiable.

• 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.

• 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.

• Short Body Design: The 1/2-inch length and 1/16-inch grip make this bolt ideal for securing thin materials and components in tight spaces.

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

Technical Specifications

• Part Number: AN4H4A

• Component Type: Drilled Head Hexagonal Bolt

• Governing Specification: AN4H / NASM4H

• 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): 1/2 inch (0.500 inches)

• Grip Length (Unthreaded Shank): 1/16 inch (0.0625 inches)

• Thread Specification: 1/4-28 UNF-3A

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

Aircraft Compatibility

The AN4H4A 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. 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

• Powerplant: Securing engine accessories, magnetos, vacuum pumps, engine baffles, and exhaust system brackets.

• Airframe: Attaching structural brackets, inspection panels, and system components located in areas of high vibration.

• Systems: Mounting hydraulic components, filter housings, and fluid line clamps.

Why Choose AN4H4A?

In aviation, there is no substitute for certified hardware. An AN4H4A 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 lower tensile strength, poor fatigue life, and susceptibility to corrosion. This certified fastener, with its provision for positive locking, ensures the joint will remain secure under the most demanding operational conditions.

The engineering principle of using an AN4H4A with safety wire is to create a "positive lock" that leverages tensile strength to prevent rotation. When safety wire (e.g., MS20995C32) is installed using the double-twist method between two or more AN4H4A bolts, it is routed in such a way that the wire itself is placed in tension. Any tendency for a bolt head to rotate in the loosening direction (counter-clockwise) results in pulling on the wire, which further increases its tension. Since a wire is exceptionally strong against tensile forces, this action effectively resists and arrests the loosening torque. The wire is always routed from one bolt to the next so that the pull on the wire exerts a tightening force on the fastener head, creating an elegant, simple, and visually verifiable locking system that is one of the most reliable in all of mechanical engineering.

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 new, clean, properly tensioned, and twisted correctly, with the pigtail bent back to prevent injury.

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.