AN177-23 – Aerospace AN Close Tolerance Bolt 7/16-20
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Overview
The AN177-23 is a military-standard, close-tolerance structural bolt, a flight-critical fastener engineered for high-stress aerospace applications where joint precision and shear strength are paramount. Unlike standard AN bolts, the AN177 series features a shank that is machined to a much tighter and more precise dimensional tolerance (typically 0.0005" undersize from nominal). This "close tolerance" fit is designed to create a near line-to-line condition when installed in a precision-reamed hole, preventing joint slippage and ensuring that shear loads are transferred directly through the bolt's shank. It is a fundamental component in building and repairing critical airframe structures.
Key Features
- Close Tolerance Shank: The shank diameter is ground to an exacting tolerance, ensuring a snug, interference-like fit in precision-reamed holes. This minimizes movement within the joint, dramatically increasing the fatigue life and rigidity of the structure.
- High-Strength Steel Alloy: Fabricated from high-grade alloy steel (such as 4130 or 4140) 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 undrilled portion of the shank (grip length) is designed to bear the full shear load, and the tight fit ensures loads are transferred without shock or play.
- Drilled Shank: Features a hole drilled in the threaded portion of the shank to accommodate a cotter pin, providing a secure, positive locking mechanism when used with a castellated nut (like an AN310 or AN320).
- Cadmium Plating: Plated with cadmium for superior corrosion resistance, ensuring the bolt maintains its structural integrity throughout its service life, even in harsh environmental conditions.
Technical Specifications
- Standard: AN177 (Bolt, Hex Head, Close Tolerance, Alloy Steel)
- Full Part Number: AN177-23
- Material: High-Tensile Alloy Steel (e.g., 4130, 4140)
- Finish: Cadmium Plated
- Head Style: Hexagonal (Hex Head)
- Nominal Diameter (-7): 7/16 inch (0.4375 inches)
- Shank Diameter: Close Tolerance (e.g., 0.4365" - 0.4370")
- Grip Length (Unthreaded Shank): 1-13/16 inches (1.8125 inches)
- Overall Length (-23): 2-25/64 inches (2.391 inches)
- Thread Specification: 7/16-20 UNF-3A
- Hole for Cotter Pin: Yes
Aircraft Compatibility
Close tolerance bolts like the AN177-23 are not specific to a single aircraft model but are used in any critical structural joint where the original manufacturer's engineering drawings call for a precise fit to carry shear loads.
- Common Applications:
- Wing Spar Attach Points: Securing wing spars to the fuselage structure.
- Engine Mounts: Fastening the engine mount assembly to the firewall.
- Landing Gear Trunnions: Used in the pivotal joints of main and nose landing gear.
- Flight Control Surfaces: Attaching hinges for ailerons, elevators, and rudders where zero play is desired.
Applications
The AN177-23 is specifically chosen for the most demanding structural joints:
- Shear-Critical Joints: Its primary application is in joints that transfer loads through shear forces, such as clevis fittings and structural lugs.
- Reamed-Hole Installations: Designed for installation in holes that have been precision-reamed to a specific final diameter to match the bolt's tight tolerance.
- Fatigue-Critical Structures: The tight fit prevents joint slippage, which is a primary cause of cyclical stress and fatigue failure in standard bolted joints.
Why Choose AN177-23?
In an airframe structure, the difference of a few thousandths of an inch can be the difference between a safe joint and a fatigue-prone one. The AN177-23 close tolerance bolt is an engineered solution that provides a level of precision far beyond standard hardware. Using a genuine AN177 bolt ensures the joint will behave as the aircraft designer intended—transferring loads efficiently and safely without movement. Substituting a standard AN7 bolt in an application that calls for an AN177 is a serious safety violation that can compromise the structural integrity of the aircraft.
Order Your AN177-23 Bolts Now
Ensure the structural integrity of your aircraft with certified, precision-engineered hardware. Visit Airparts.aero to order your AN177-23 close tolerance bolts today.
The fundamental engineering principle behind the AN177 close tolerance bolt is to create a "bearing-type" connection where the bolt shank bears directly against the wall of the hole to transfer shear loads. This is in contrast to a standard "friction-type" connection where clamping force from bolt tension creates friction between the plates to resist slip. To achieve a proper bearing connection, the hole must be drilled undersize and then reamed to a precise diameter to create a "transition fit" with the close-tolerance shank. This fit is snug enough to prevent any significant play or movement, ensuring that when a shear load is applied, it is immediately borne by the bolt's cross-sectional area, minimizing stress concentrations and maximizing the joint's fatigue life.
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 and proper torquing of the corresponding AN310 or AN320 castellated nut, secured with a cotter pin. Any substitution is prohibited.