AN3-21 – NASM3 Specification Undrilled Shank Machine Bolt
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The AN3-21 is a military-specification, flight-grade Undrilled Shank Machine Bolt manufactured in strict accordance with the NASM3 (formerly AN3) standard. This component is a foundational piece of certified aerospace hardware, engineered for critical structural applications in both tension and shear. With a 3/16-inch diameter and a precise 1-5/16 inch grip length, the AN3-21 is designed to clamp materials of a specific thickness, ensuring that shear loads are borne by the full-diameter, unthreaded shank, thereby maximizing joint strength and fatigue life. Its solid, undrilled shank designates it for use where maximum shear strength is paramount and locking is achieved via a self-locking nut.
As a fundamental building block in airframe and powerplant assembly, the AN3-21 bolt represents a commitment to uncompromising quality and traceability. Forged from high-tensile alloy steel and protected by a sacrificial cadmium plating, it provides the requisite strength, ductility, and corrosion resistance demanded by the harsh aerospace environment. The precise manufacturing of its 10-32 UNF-3A threads ensures proper load distribution and resistance to vibrational loosening when torqued to specification, making it a trusted component for A&P mechanics and aerospace engineers globally.
Key Features
NASM3 Certified: Manufactured and inspected to the rigorous National Aerospace Standard, ensuring complete airworthiness and traceability.
High-Tensile Steel Core: Forged from high-strength alloy steel (typically 4130 or 8740) for superior tensile and shear performance.
Full-Diameter Undrilled Shank: Provides maximum shear strength by eliminating the stress-concentrating hole found in drilled-shank bolts, improving fatigue life.
Precision Rolled Threads: Features Class 3A fit threads (10-32 UNF), offering the tightest tolerance for secure fastening and optimal load distribution.
Superior Corrosion Resistance: Protected by a uniform cadmium plating for galvanic compatibility and robust protection against environmental corrosion.
Positive Head-to-Shank Fillet: A radiused fillet at the head-to-shank junction reduces stress concentration and improves fatigue resistance under cyclic loading.
Technical Specifications
Part Number: AN3-21
Alternate Part Number / Specification: NASM3-21
Component Type: Hex Head Undrilled Shank Machine Bolt
Material: Alloy Steel (e.g., AISI 4130 or AISI 8740) heat-treated to 125,000 PSI minimum tensile strength.
Finish: Cadmium Plate per QQ-P-416, Type II, Class 3.
Head Style: Hexagonal (HEX)
Nominal Diameter: 3/16 inch (0.1875 in)
Grip Length (Shank): 1-5/16 inch (1.3125 in)
Overall Length: 1-11/16 inch (1.688 in)
Thread Specification: 10-32 UNF-3A
Thread Length: 0.359 inches
Aircraft Compatibility
The AN3 series bolt is a universally specified fastener used in thousands of applications across a vast range of aircraft. Its use is not specific to a single model but is dictated by the design requirements of the sub-assembly.
Applicability: Widely used in certified piston and turbine aircraft (Cessna, Piper, Beechcraft), experimental amateur-built (EAB) aircraft, and vintage/military aircraft.
Note: The aircraft's specific Illustrated Parts Catalog (IPC) or maintenance manual is the sole authoritative document for verifying part number applicability and proper installation location.
Applications
The AN3-21 is a general-purpose airframe bolt designed for structural joints. Its full, undrilled shank makes it ideal for:
Shear Joints: Attaching structural brackets, mounting plates, and support members where the primary load is perpendicular to the bolt's axis.
Tension Joints: Clamping components together, such as engine baffling, fairings, access panels, and instrument mounts.
Control System Assemblies: Used in linkages and attachments where a close-tolerance fit and high strength are required.
Why Choose the NASM3-Certified AN3-21?
In aviation, there is no substitute for certified hardware. The NASM3-certified AN3-21 bolt guarantees that the material composition, heat treatment, manufacturing tolerances, and protective finish have been rigorously controlled and inspected. Using uncertified or commercial-grade hardware introduces a catastrophic risk of failure due to incorrect tensile strength, improper thread geometry, or susceptibility to hydrogen embrittlement from improper plating. Sourcing a fully traceable, certified AN3-21 bolt is a fundamental requirement for any airworthy repair or assembly, ensuring the safety and structural integrity of the aircraft.
The engineering superiority of the AN3-21 bolt for aerospace joints lies in the metallurgical and mechanical principles dictated by the NASM3 standard. The component is forged from a medium-carbon, low-alloy steel like 4130, which is then normalized, quenched, and tempered to achieve a fine-grained martensitic microstructure. This provides an optimal balance of ultimate tensile strength (UTS) and fracture toughness, preventing brittle failure under shock loads. Critically, the threads are rolled, not cut, after heat treatment. This cold-working process induces compressive residual stresses at the thread root—the most common point of fatigue crack initiation—significantly increasing the bolt's fatigue life under vibratory and cyclic loading. The unthreaded shank's diameter is held to a tight tolerance, allowing it to act as a close-fit dowel in shear applications, preventing joint slippage and ensuring load is transferred through bearing stress, not just friction between clamped members.
Note: The AN3-21 is a flight-critical structural fastener. Installation must be performed by a certified A&P mechanic. The bolt must be installed with the correct mating nut (e.g., AN365 or MS21044) and washer (e.g., AN960) and torqued to the value specified in the aircraft's Structural Repair Manual (SRM) or standard reference documents like AC 43.13-1B. Overtorquing can lead to plastic deformation and failure, while under-torquing can result in joint failure due to fatigue.
Documentation
No public-facing OEM or regulatory documentation was located for this part. The full NASM3 specification is a controlled document, but public-domain reference sheets detailing the dimensional and material standards are widely available from aerospace hardware suppliers.