AN4-13 - Aerospace Drilled Machine Bolt

AN4-13 – Aeronautical Standards Group Drilled Aerospace Machine Bolt

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The AN4-13 is a flight-critical, high-strength structural airframe machine bolt manufactured in strict accordance with the Aeronautical Standard Group (AN) guidelines and MIL-DTL-6812 military standard specifications. Featuring a 1/4-28 UNF-3A fine thread profile and a nominal shank diameter of 1/4 inch ($0.250\text{ in}$), this precision hex-head fastener is explicitly engineered to handle high-load structural joints across aircraft skins, control surface hinges, wing attach points, and engine mount assemblies. The absence of an "A" suffix designates a drilled shank configuration, featuring a precision-machined hole through the threaded section to accept a cotter pin or safety wire, establishing a positive mechanical lock required for high-vibration applications.

Fabricated from premium low-alloy structural steel—typically 8630, 8740, or 4130 chrome-molybdenum alloy—the AN4-13 undergoes controlled heat treatment to achieve a minimum tensile strength of 125,000 psi ($861\text{ MPa}$). The bolt is finished with an electro-deposited cadmium plating (QQ-P-416 Type II, Class 2) topped with a gold chromate conversion coating. This plating acts as a sacrificial anode layer, preventing surface oxidation and mitigating galvanic corrosion when the steel fastener is driven into dissimilar non-ferrous aluminum or magnesium airframe components. Featuring a precision-ground, unthreaded grip length of 29/32 inches ($0.906\text{ in}$), this bolt ensures that the primary transverse shear forces within a structural joint rest entirely against the solid, full-diameter shank rather than the weaker, notched threaded plane.

The AN4-13 represents the baseline hardware standard for general aviation, commercial aircraft fleet maintenance, and defense airframe sustainment. Its structural profile is precision-forged to eliminate internal microscopic stress concentrations and micro-voids, providing highly predictable fatigue life limits under alternating aerodynamic loads and severe vibration profiles. For structural repair engineers, airframe mechanics, and aerospace procurement specialists, sourcing fully traceable AN4-13 hardware guarantees that airframe joints match the exact original type-certificate documentation, preventing premature fastener shearing and structural joint relaxation under heavy flight dynamics.

Key Features

• High-Tensile Alloy Steel Matrix: Heat-treated to standard aerospace specifications, yielding a minimum tensile strength of 125,000 psi to ensure exceptional load-bearing safety margins.

• Positive Locking Drilled Shank: Precision-machined 1/16-inch cross-hole near the chamfered end accepts standard cotter pins (e.g., MS24665) to eliminate any risk of unfastening under cyclic vibration.

• Optimized Shear Grip Length: Engineered with an unthreaded 29/32-inch shank area designed to take the brunt of primary shear loads, preventing internal thread-bearing deformation within the joint.

• Sacrificial Cadmium Plating: Plated per QQ-P-416 parameters to maximize defense against atmospheric oxidation and eliminate galvanic corrosion when interfacing with non-ferrous structures.

• Precision Rolled Threads: Thread profiles are rolled rather than cut after heat treatment, which preserves material grain continuity and maximizes resistance to fatigue-crack propagation.

Technical Specifications

• Part Number: AN4-13

• Component Type: Hex-Head Drilled Shank Structural Aerospace Bolt

• Governing Specification: AN4 Standard / MIL-DTL-6812 (Superseded by NASM6812)

• Thread Size / Pitch: 1/4-28 UNF-3A (Fine Thread)

• Nominal Shank Diameter: 1/4 Inch ($0.250\text{ in}$)

• Grip Length (Unthreaded Portion): 29/32 Inch ($0.906\text{ in}$)

• Total Length (Under Head to Tip): 1-17/64 Inches ($1.266\text{ in}$)

• Drilled Hole Diameter: 0.076 Inches ($0.076\text{ in}$ nominal / designed for 1/16" cotter pins)

• Head Configuration: Standard Hexagon Flat Head

• Width Across Flats (Wrench Size): 7/16 Inch

• Material Composition: Heat-Treated Medium Carbon / Low-Alloy Steel (AISI 4130 / 8630 / 8740 per MIL-S-6758)

• Protective Finish: Cadmium Plated (QQ-P-416 Type II, Class 2) with Chromate Conversion

• Minimum Tensile Strength: 125,000 psi ($861\text{ MPa}$)

• Minimum Shear Strength: 75,000 psi ($517\text{ MPa}$)

• Certification Status: Certificate of Conformance (CoC) / Full Material Traceability Profile

Aircraft Compatibility

• General Aviation Fleet: Cessna 152, 172 Skyhawk, 182 Skylane, 206 Stationair, 210 Centurion Series; Piper PA-28 Cherokee, PA-32 Saratoga, PA-34 Seneca, PA-44 Seminole Series; Beechcraft Bonanza, Baron, Duke, and King Air 90/200/350 Series.

• Commercial & Transport Airframes: Regional turboprops, utility transports, and light commercial cargo platforms requiring standard structural hardware additions.

• Experimental & Light Sport Aircraft: Vans RV series (RV-4, RV-6, RV-7, RV-8, RV-10, RV-14), Kitfox, and specialized amateur-built airframes.

• Note: Fastener selection, edge-margin limits, and installation authority must be verified strictly by referencing the target aircraft's specific Illustrated Parts Catalog (IPC), Structural Repair Manual (SRM), or approved standard hardware logs.

Applications

• Structural Airframe Joints: Securing primary skin sheets, fuselage bulkheads, wing-to-fuselage attach brackets, interior floor grid paths, and landing gear attachment hardware.

• Flight Control Linkages: Providing the primary pivot and connection bolts for non-rotating linkages, push-pull rods, torque tubes, and bellcrank bracket mounts.

• Engine Accessory Attachments: Mounting secondary fluid line brackets, cooling baffles, fuel-line clamps, and electrical grounding straps within the engine cowl environment.

Why Choose AN4-13?

Selecting a trace-certified AN4-13 structural bolt prevents the catastrophic joint failures associated with commercial-grade or non-aviation fasteners. Standard automotive or industrial grade-8 bolts lack the structural grain-flow uniformity, precise grip tolerances, and rigorous non-destructive testing (NDT) criteria mandatory for flight operations. Commercial fasteners driven into an aircraft joint frequently manifest inconsistent grip-to-thread ratios, causing the threaded plane to bear internal shear loads. This misapplication can induce rapid fatigue-crack propagation, thread stripping, and total structural joint failure under high-G aerodynamic maneuvers. The pre-drilled cross-hole of the AN4-13 allows for a positive mechanical lock via a cotter pin or safety wire, ensuring that even if torque values degrade due to extreme thermal cycling, the nut cannot back off the threads, preserving structural safety margins.

The structural performance of the AN4-13 bolt depends on the mechanical principles of elastic clamping elongation and pure transverse shear stress distribution across structural joint interfaces. When torque is applied during installation, the 1/4-28 UNF rolled threads engage the matching nut threads, converting rotational torque into a controlled linear tension vector along the bolt's longitudinal axis. This action stretches the bolt within its elastic deformation region, creating a permanent pre-load or clamp force that prevents joint separation under cyclic loading. To withstand high-frequency aerodynamic vibration without inducing fatigue-crack propagation, the bolt features an isotropic granular grain flow created during the head-forging and thread-rolling processes. This continuous structural grain pattern eliminates the microscopic stress concentrations common to cut threads. The 29/32-inch unthreaded grip length is sized precisely to prevent the joint's mating shear planes from resting against the threaded region. Keeping the shear load on the solid shank capitalizes on the material's maximum cross-sectional area, avoiding the concentrated notch filters that trigger accelerated fatigue fractures and premature fastener failure under heavy flight dynamics.

Note: The AN4-13 is a structural, flight-governing fastener. Undersized grip applications, driving the threads into the joint's primary shear path, or over-torquing can deform the thread root, causing immediate loss of clamp load, fastener shearing, or structural joint failure. Fastener installation, stack-up washer configuration, and assembly inspection must be performed exclusively by a certified A&P/IA mechanic in strict compliance with the aircraft structural repair manual (SRM), standard practices manual, and FAA Advisory Circular AC 43.13-1B Chapter 7. Always verify that a minimum of one to three threads extend past the nut element, and ensure the correct thickness of washers (e.g., AN960) is utilized so the cotter pin hole aligns correctly with the castle nut slots without placing the threads into a bearing shear condition.

Documentation

• FAA Advisory Circular AC 43.13-1B: Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair (Chapter 7: Aircraft Hardware)

• Assist-Quicksearch MIL-DTL-6812 Specification Sheet Index

OVERVIEW

Hex head aircraft bolts are made of high-strength type 4037 or 8740 alloy steel (type 8740 is most commonly used). The bolts are centerless ground and threaded after heat treatment. Minimum tensile strength 125,000 PSI. Cadmium plated per specification QQ-P-416A, Type II, Class 3. Available with shank drilled for cotter pin or undrilled for stop nut application, and with or without drilled head for safety wire. Specify bolts to have undrilled shank by adding letter "A" after the dash number. For bolts with drilled head add letter "H" after the AN number. See illustrated examples. "C" represents Type 431 Stainless Steel (contains iron which is magnetic). The length of AN aircraft bolts is measured from under the head to the end of the shank. The "grip" is the unthreaded portion of the shank. See table for conversion of length and/or grip to proper AN callout.

Product Dimension: Length: 1-13/32, Grip: 15/16", Thread: 1/4-28
Cadmium plated steel, drilled shank

AN4-13  Datasheet Features and Specification

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