AN4-12A - Undrilled Shank Machine Bolt

AN4-12A – Aeronautical Standards Group Undrilled Machine Bolt

1/4-28, UNF, 3A, Right Hand, 0.468 in L Thread

Description:   BOLT - Undrilled Shank
Part number:   AN4-12A 
Alternate part number:   Beech substitute - 130909B41 

Product Overview:

An externally threaded fastener designed for insertion through holes in assembled parts and normally intended to be tightened or released by torquing a nut. the threaded and unthreaded portions are both of one nominal diameter of 0.190 inch (5 mm) or larger. The head is not designed to be held or driven with an inserted driver or by the thumb and fingers. a locking feature may be incorporated in the design of the head or threads. head dimensions and sizes of threaded and unthreaded portions shall conform to internationally recognized fastener standards. for items having a grip length portion machined to a tolerance of 0.0015 inch (0.038 mm) or less, bolt, shear. See also screw, cap, hexagon head and screw, machine.

Product Features and Specification:

Available now on the Airparts.aero Marketplace.

The AN4-12A 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 thread profile and a nominal shank diameter of 1/4 inch, this precision hex-head fastener is specifically engineered to handle high-load structural joints across airframe skins, flight control linkages, and primary engine mount assemblies. The "A" suffix designates an undrilled shank configuration, which makes it the standard choice for applications utilizing self-locking prevailing-torque fasteners (such as MS21042 or MS21044 series nuts) where traditional safety-wire or cotter-pin assemblies are not required or specified.

Fabricated from premium low-alloy structural steel—typically 8630, 8740, or 4130 chrome-molybdenum alloy—the AN4-12A 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 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 25/32 inches, this bolt ensures that the primary shear forces within a joint rest entirely against the solid, full-diameter shank rather than the weaker, notched threaded plane.

The AN4-12A represents the baseline hardware standard for general aviation, commercial aircraft fleet maintenance, and defense airframe upkeep. 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 AN4-12A hardware guarantees that airframe repairs match the exact original type-certificate documentation, preventing premature fastener shearing and structural joint relaxation.

Key Features

• High-Tensile Alloy Steel Matrix: Heat-treated to a minimum tensile strength of 125,000 psi, ensuring exceptional load-bearing safety margins for primary structural tie-ins.

• Optimized Shear Grip Length: Engineered with an unthreaded 25/32-inch shank area designed to take the brunt of primary shear loads, preventing internal thread-bearing deformation.

• Sacrificial Cadmium Plating: Coated to maximize resistance to atmospheric oxidation and eliminate galvanic corrosion when interfacing with non-ferrous structures.

• Undrilled Shank Layout: Standard flat-hex configuration without safety-apertures, optimized for rapid torque installation when paired with all-metal or nylon prevailing-torque locking nuts.

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

Technical Specifications

• Part Number: AN4-12A

• Component Type: Hex-Head Structural Airframe Machine Bolt

• Governing Specification: AN4 Standard / MIL-DTL-6812

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

• Nominal Shank Diameter: 1/4 Inch (0.250 in)

• Grip Length (Unthreaded Portion): 25/32 Inch (0.781 in)

• Total Length (Under Head to Tip): 1-9/64 Inches (1.141 in)

• Head Configuration: Standard Hexagon Flat Head

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

• Material Composition: Heat-Treated Medium Carbon / Low-Alloy Steel (e.g., 4130 / 8630 / 8740)

• 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 Basis: Certificate of Conformance (CoC) / Full Material Traceability Documentation

Aircraft Compatibility

• General Aviation Fleet: Cessna 152, 172 Skyhawk, 182 Skylane, 210 Centurion Series; Piper PA-28 Cherokee, PA-32 Saratoga, PA-34 Seneca, PA-44 Seminole Series; Beechcraft Bonanza, Baron, Duke 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, interior floor grid paths, and exterior fairing brackets.

• Flight Control Linkages: Providing the primary pivot 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 AN4-12A?

Selecting a trace-certified AN4-12A 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. AN4 series hardware ensures strict dimensional parallelism, providing predictable clamp loads and lasting shear capabilities throughout its lifetime.

The structural performance of the AN4-12A bolt depends on the mechanical principles of elastic clamping elongation and pure 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 25/32-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 AN4-12A 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 locking nut element and apply the exact dry or lubricated torque values specified for cadmium-plated hardware.

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

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