AN175H22A – Close Tolerance Aerospace Machine Bolt
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Overview
The AN175H22A is a genuine, military-standard, flight-critical close tolerance bolt, an advanced structural fastener designed for high-shear, high-stress aerospace applications where both precise fit and positive locking are required. This is not a standard airframe bolt; the AN175 series features a shank that is precision-ground to a much tighter dimensional tolerance, while the "H" designator signifies a drilled head for securing with safety wire. Fabricated from high-tensile strength alloy steel, the AN175H22A is engineered to create a near-perfect, load-bearing fit in reamed holes, preventing joint slippage and providing a visually verifiable locking method.
This combination of a close-tolerance shank and a drilled head for safety wiring makes the AN175H22A a specialized fastener for the most demanding structural joints, such as flight control surfaces and engine mounts, where the integrity of the connection is paramount to the safety of flight.
Key Features
Close Tolerance Shank: The bolt's shank diameter is ground to an exacting tolerance (e.g., 0.0005" undersize from nominal). This ensures a snug, interference-like fit in precision-reamed holes, which minimizes movement, increases the joint's fatigue life, and allows the shank to bear shear loads directly.
Drilled Head for Safety Wiring: The primary safety feature is a hole drilled through the flats of the hexagonal head. This allows for the installation of safety wire, a positive locking method that physically prevents the bolt from rotating or loosening due to vibration.
High-Strength Steel Alloy: Fabricated from high-grade alloy steel (typically 4140 or 8740) and heat-treated to a minimum tensile strength of 125,000 PSI, providing the strength required for primary structural applications.
Shear Load Design: Specifically intended for applications where forces act perpendicular to the bolt's axis. The close-tolerance, unthreaded portion of the shank (grip length) is designed to bear the full shear load without play.
Undrilled Shank ("A"): The "A" suffix signifies that the threaded portion of the shank is undrilled. This bolt is intended for use with a self-locking nut, with the safety wire on the head providing a redundant or primary positive locking feature.
Technical Specifications
Standard: AN175H (Bolt, Hex Head, Drilled Head, Close Tolerance, Alloy Steel)
Full Part Number: AN175H22A
Material: High-Tensile Alloy Steel (e.g., 4140)
Finish: Cadmium Plated
Head Style: Drilled Hexagonal (Hex Head)
Nominal Diameter (-5): 5/16 inch (0.3125 inches)
Shank Diameter: Close Tolerance (e.g., 0.3115" - 0.3120")
Overall Length (-22): 2-1/4 inches (2.250 inches)
Grip Length (Unthreaded Shank): 1-13/16 inches (1.8125 inches)
Thread Specification: 5/16-24 UNF-3A
Drilled Shank: No ("A" Suffix)
Aircraft Compatibility
Close tolerance bolts like the AN175H22A are specified in any critical structural joint where the original manufacturer's engineering drawings demand a precise fit and a positive locking method to carry shear loads.
Common Applications:
Flight Control Systems: Attaching hinges and bellcranks for ailerons, elevators, and rudders where zero play is required for a positive control feel.
Engine Mounts & Accessories: Securing engine mounts and high-vibration engine accessories to the firewall or engine case.
Landing Gear Trunnions: Used in the primary pivotal joints of main and nose landing gear assemblies.
Primary Airframe Structure: Used in wing spar and other critical structural joints where joint slippage is unacceptable.
Why Choose AN175H22A?
In an airframe structure, a standard bolt in a standard hole can allow for microscopic movement, which, over millions of cycles, can lead to hole elongation and fatigue failure. A close-tolerance bolt prevents this. The addition of a drilled head for safety wire provides a virtually infallible locking system. Using a genuine AN175H22A is the only way to ensure the fastener has the certified precision, material strength, and design features required for the most critical joints on an aircraft.
The AN175H22A combines two advanced fastening principles for ultimate joint security. The close-tolerance shank creates a "bearing-type" connection. To achieve this, the receiving hole is drilled undersize and then precision-reamed to a final diameter that creates a near-interference fit with the bolt's shank. This ensures that when a shear load is applied, it is immediately borne by the full cross-section of the bolt shank without any initial slippage, which is the primary driver of joint fatigue. The drilled head provides a "positive lock." Unlike a friction-based locknut, safety wire creates a physical barrier that makes it impossible for the bolt to rotate loose. This combination of a precision fit to eliminate movement and a physical lock to prevent rotation creates one of the most secure and reliable structural joints used in aerospace engineering.
Note: This is a flight-critical structural component. It must only be used where specified by the aircraft manufacturer's maintenance or structural repair manual. Installation requires precision-reamed holes, the use of a calibrated torque wrench with a corresponding certified nut, and proper safety wiring technique in accordance with FAA AC 43.13-1B. Any substitution is strictly prohibited.