NAS1351-3-8P - Cadmium Socket Head Cap Aerospace Screw

NAS1351-3-8P – High-Strength Socket Head Cap Screw

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Overview

The NAS1351-3-8P is a National Aerospace Standard (NAS), ultra-high-strength, structural socket head cap screw. This is a flight-critical fastener engineered for the most demanding aerospace applications where a combination of extremely high tensile strength, superior fatigue life, and limited clearance are required. Unlike a standard AN bolt, the NAS1351 series is heat-treated to a minimum tensile strength of 160,000 PSI. Its internal-wrenching socket head design allows for high torque application in confined spaces where a traditional external hex head would not fit.

This fastener is the product of the most advanced manufacturing processes. It features a forged head, a precision-machined shank, and, most critically, threads that are rolled after heat treatment. This process induces compressive stresses that dramatically increase the screw's resistance to fatigue failure. Often featuring a UNJ thread profile with a controlled root radius, this screw is designed from the ground up to provide the ultimate in strength and reliability for primary structural joints, such as those found in landing gear and engine mounts.

Key Features

• NAS Certified: Manufactured and tested to the stringent requirements of National Aerospace Standard NAS1351, guaranteeing material, dimensional, and strength specifications.

• 160 KSI High-Tensile Strength: Fabricated from a premium alloy steel and heat-treated to achieve a minimum tensile strength of 160,000 PSI, far exceeding standard AN bolts.

• Superior Fatigue Life: Features threads that are rolled after heat treatment and a UNJ-profile with a controlled root radius, a combination that provides the maximum possible resistance to fatigue failure under cyclic loading.

• Internal Wrenching Socket Head: Allows for high torque application and is ideal for use in counterbored holes or areas with tight radial clearance.

• Forged Head for Superior Strength: The head is forged to align the metal's grain structure, providing superior strength and resistance to shear and fatigue at the head-to-shank junction.

• Cadmium Plated: The "P" suffix designates a Type II cadmium plating per QQ-P-416 for exceptional, long-lasting corrosion protection.

Technical Specifications

• Part Number / Specification: NAS1351-3-8P

• Component Type: Screw, Cap, Socket Head, Alloy Steel

• Thread Size (-3): #10-32 UNF/UNJ-3A

• Grip Length (-8): 8/16 inch (1/2" or 0.500")

• Material: High-Strength Alloy Steel, heat-treated to 160-180 KSI tensile strength.

• Finish (P): Cadmium Plated, Type II (Yellow Chromate) per QQ-P-416.

• Head Type: Socket Head (Internal Hex Drive), Undrilled.

• Governing Specification: NAS1351.

Aircraft Compatibility

• Universal High-Performance Application: This is a premier structural fastener used in the assembly of virtually all modern, high-performance military, commercial, and business aircraft from all major OEMs, including:

  • Textron Aviation, Boeing, Airbus, Lockheed Martin, Bombardier, Gulfstream, and others.

Note: The correct fastener for any structural application must be verified against the aircraft's specific Structural Repair Manual (SRM) or engineering drawings.

Applications

The superior strength and fatigue resistance of the NAS1351 screw make it essential for the most critical, high-load joints on an aircraft:

• Primary Airframe Structure: Used in wing-to-fuselage attachments, spar connections, and bulkhead installations.

• Landing Gear Systems: Securing main landing gear trunnions, actuators, torque links, and wheel/brake assembly components.

• Engine Mounts & Pylons: The fastener of choice for attaching the engine and its pylon to the airframe structure.

• Flight Control Surfaces: Attaching the hinges and actuator fittings for primary flight controls like flaps, elevators, and stabilizers.

Why Choose NAS1351-3-8P?

There is no substitute for a NAS-certified structural fastener. Using a standard AN bolt or, even worse, a commercial-grade industrial socket head cap screw in an application that specifies a NAS1351 is a catastrophic risk. An industrial screw will have a fraction of the tensile strength, will be brittle from having its threads cut after heat treat, and will lack any of the fatigue-resistant design features. It is guaranteed to fail under the cyclic loads of flight. The genuine NAS1351-3-8P is an engineered system—the culmination of advanced metallurgy, manufacturing, and design—built to ensure the absolute structural integrity of the aircraft in its most critical joints.

The exceptional fatigue life of the NAS1351-3-8P is primarily achieved through a combination of post-heat-treat thread rolling and the UNJ thread profile. Fatigue cracks in fasteners almost invariably initiate at the root of the first engaged thread, a point of high stress concentration. The process of rolling the threads after the screw has been hardened to 160 KSI is a severe cold-working operation. It plastically deforms the material at the thread root, creating a smooth, flowing grain structure and, most importantly, inducing a deep layer of high-magnitude compressive residual stress. This compressive stress acts as a protective shield; the tensile loads of the application must first overcome this built-in compressive stress before the thread root sees any net tension. This dramatically increases the number of load cycles the screw can endure before a fatigue crack can initiate. The UNJ thread profile, with its larger, controlled root radius, further reduces the stress concentration factor, working in synergy with the rolled threads to create a fastener with the maximum possible fatigue life.

Note: The NAS1351-3-8P is a flight-critical structural fastener. It must be installed by a certified A&P/IA mechanic with structural experience. Installation requires a specific, high torque value to develop the correct clamp load (preload). A calibrated torque wrench is mandatory. The internal hex socket and the tool bit must be clean and in perfect condition to allow full engagement and prevent stripping the head during high-torque application.

Documentation

• NAS1351: The National Aerospace Standard document defines all dimensional, material, and performance requirements for this series of screws and is available through various standards providers.