NAS6604-44 - Military Standard Close Tolerance 12-Point Shear Bolt

NAS6604-44 – Military Standard Close Tolerance 12-Point Shear Bolt

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The NAS6604-44 is a flight-critical, high-strength, close-tolerance shear bolt manufactured in strict accordance with National Aerospace Standard (NAS) 6604. This fastener is specifically engineered for use in demanding aerospace structural joints where the primary loads are in shear, and where precise fit and high fatigue life are paramount. It features a 12-point (double hexagon) external wrenching head for high torque application and a shank diameter controlled to extremely tight tolerances for an interference or close-clearance fit in precision-reamed holes.

Fabricated from high-strength alloy steel (typically 4340 or similar), heat-treated to 160-180 KSI tensile strength, and finished with cadmium plating for corrosion resistance, the NAS6604-44 is designed for maximum performance in critical applications. The shear design often incorporates a reduced shank diameter compared to tension bolts, optimizing its strength-to-weight ratio for shear loading. The use of certified NAS close-tolerance hardware is essential for ensuring the structural integrity and fatigue life of aircraft components subjected to high shear stresses.

Key Features

• National Aerospace Standard (NAS) Certified: Guarantees adherence to stringent material, dimensional tolerance (close tolerance shank), heat treatment, and quality requirements.

• Designed for Shear Loading: Optimized geometry and material properties for joints where loads act primarily parallel to the mating surfaces.

• High-Strength Alloy Steel (160-180 KSI): Manufactured from heat-treated alloy steel (e.g., 4340 per MIL-S-5000) for exceptional strength and fatigue resistance.

• Close Tolerance Shank: Shank diameter is precision ground to tight tolerances (typically +0.0000 / -0.0005 inches of nominal) for a near-interference fit, improving load transfer and joint fatigue life.

• 12-Point External Wrenching Head: Allows for high installation torque application and provides superior tool engagement compared to standard hex heads, especially in restricted access areas.

• Cadmium Plated for Corrosion Protection: Coated with cadmium per QQ-P-416 for excellent, long-lasting corrosion resistance.

Technical Specifications

• Part Number: NAS6604-44

• Component Type: Bolt, Shear, Close Tolerance, 12-Point External Wrenching

• Governing Specification: NAS6604

• Material: Alloy Steel, Heat Treated to 160-180 KSI Tensile Strength (e.g., 4340).

• Finish: Cadmium Plated per QQ-P-416, Type II, Class 2 or 3.

• Dimensions:

  • Nominal Diameter: -4 (1/4 inch / 0.2500 in)

  • Shank Tolerance: Close Tolerance (e.g., 0.2495 - 0.2500 inches)

  • Length (Nominal, under head to end): -44 (44/16 inch = 2-12/16 = 2-3/4 inches / 2.750 in)

  • Threads: 1/4-28 UNF-3A

  • Head Style: 12-Point External Wrenching

• Minimum Shear Strength: Typically 95 KSI (Verify with NAS6604 standard).

Aircraft Compatibility

The NAS6604-44 is a standard hardware component used in critical structural joints across all categories of high-performance aircraft, including military, commercial transport, and business jets from all major manufacturers (e.g., Textron, Boeing, Airbus, Lockheed Martin, Gulfstream).

Crucial Note: The correct bolt part number (including material, diameter, length, and grip length) for a specific application must be verified against the aircraft's Structural Repair Manual (SRM), Illustrated Parts Catalog (IPC), or engineering drawings.

Applications

This bolt is specifically designed for high-load shear joints where close tolerances are required to maximize fatigue life and ensure proper load distribution. Common applications include:

• Wing Spar Attachments

• Fuselage Frame and Bulkhead Connections

• Landing Gear Trunnions and Major Fittings

• Flight Control Surface Hinge Fittings

• Engine Mount Attachments

Why Choose NAS6604-44?

Choosing a genuine NAS6604-44 bolt is absolutely critical for structural integrity in high-shear applications. These bolts operate in close-tolerance holes, often with an interference fit, which significantly improves fatigue performance by reducing relative motion between joint members. Using a standard tolerance bolt (like AN series) or a non-certified fastener in a close-tolerance hole negates these benefits and can lead to premature fatigue failure, joint loosening, or catastrophic structural failure. The NAS standard guarantees the required high-strength material, precise close-tolerance dimensions, and rigorous quality control essential for these demanding, flight-critical joints.

The engineering significance of the NAS6604-44 lies in its close-tolerance shank and design for shear loading. In a typical bolted joint designed for shear, the primary load path is through the bolt shank acting as a pin across the interface of the joined materials. A close-tolerance fit (where the hole diameter is only fractionally larger than, or even slightly smaller than, the bolt shank diameter) ensures immediate and uniform bearing contact between the bolt shank and the hole wall. This minimizes relative movement (microslip) between the parts under cyclic loading, significantly delaying the initiation of fatigue cracks and drastically increasing the fatigue life of the joint compared to clearance-fit bolts. The 12-point head design allows for higher torque transmission during installation compared to a 6-point hex head, enabling proper preload to be achieved, which further enhances fatigue performance by maintaining friction between the clamped plates. The shear bolt design often features specific head-to-shank fillet radii and thread runout characteristics optimized to withstand shear stresses without creating detrimental stress concentrations.

Note: This is a flight-critical structural fastener. It MUST be installed in a precision-reamed, close-tolerance hole prepared according to the procedures specified in the aircraft's Structural Repair Manual (SRM). Installation may require specialized techniques (e.g., lubrication, thermal methods for interference fits). Use of the correct mating high-strength nut (e.g., NAS6204) and washer is mandatory. Proper lubrication (as specified) and tightening to the precise torque value defined in the maintenance documentation are essential for achieving the designed joint performance and fatigue life.

Documentation

The definitive technical requirements for this component are detailed in National Aerospace Standard NAS6604. These proprietary documents must be sourced directly from the relevant standards organizations for complete engineering specifications.