MS17825-6 - Slotted Castellated Self-Locking Nut

MS17825-6 – 3/8-24 Castellated Self-Locking Nut (Dual-Lock)

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Overview

The MS17825-6 is a maximum-security fastener, manufactured to a stringent Military Standard for use in the most flight-critical 3/8-inch bolted joints. This component is engineered with a dual-locking system, providing two independent and dissimilar layers of protection against loosening. It integrates the friction-based, prevailing torque lock of a nylon insert with the positive, mechanical lock of a castellated nut and cotter pin. This redundant safety design makes it the mandated hardware for primary structural and flight control attachments where the consequence of fastener failure is catastrophic.

Constructed from high-strength, cadmium-plated steel, this 3/8-24 UNF nut is rated for service in temperatures up to 250°F (121°C). Its design represents the highest standard in fastener security, providing engineers and mechanics with a component that offers unparalleled resistance to the effects of vibration, thermal cycling, and g-loading.

Key Features

• Dual Redundant Locking System: Incorporates a prevailing torque nylon insert and castellated slots for a cotter pin, providing two independent layers of security against loosening.

• Prevailing Torque Nylon Insert: A captive nylon collar provides a constant frictional grip on the 3/8-24 bolt threads, maintaining crucial preload and resisting high-frequency vibration.

• Castellated for Positive Locking: The slotted design allows for the installation of a cotter pin through the nut and a drilled-shank bolt (e.g., AN6 series), creating an absolute mechanical barrier to rotation.

• High-Strength Steel Construction: Fabricated from aerospace-grade steel and plated with cadmium per QQ-P-416 for high strength and superior corrosion resistance.

• 250°F Temperature Rating: Certified for use in a wide range of demanding airframe and systems applications within its thermal limits.

• MS Standard Compliance: Guarantees every aspect of the nut—from its material and dimensions to the performance of its locking features—meets rigorous military and aerospace quality standards.

Technical Specifications

• Part Number: MS17825-6

• Component Type: Hexagon Castellated Self-Locking Nut

• Governing Specification: MS17825

• Material: Steel, Cadmium Plated

• Locking Element: Nylon Insert

• Thread Specification: 3/8-24 UNF-3B

• Maximum Operating Temperature: 250°F (121°C)

• Locking Principles:

  1. Prevailing Torque (Friction Lock)

  2. Positive Lock (Mechanical Lock via Cotter Pin)

Aircraft Compatibility

The MS17825-6 is a standard component specified on a multitude of aircraft types for their most critical connections. Its use is not limited to a single manufacturer but is common across the entire aerospace industry.

• Military Fighter, Transport, and Rotorcraft

• Commercial and Transport Category Airliners (Boeing, Airbus)

• Business and Corporate Jets (Textron Aviation, Bombardier, Gulfstream)

• High-Performance General Aviation Aircraft

Note: Verification of applicability is the responsibility of the installer. Always consult the current version of the aircraft's Illustrated Parts Catalog (IPC) and maintenance manual to confirm part number eligibility for a specific joint.

Applications

• Primary Flight Control Systems: The mandated fastener for attaching ailerons, elevators, and rudders to their hinges and actuators.

• Major Structural Joints: Used in wing-to-fuselage attachments, landing gear trunnions, and other primary structural connections.

• Engine and Pylon Mounts: Used in critical engine and powerplant attachment points.

Why Choose MS17825-6?

For the major structural applications where this 3/8-inch nut is specified, no other fastener provides a higher level of security. It is designed based on the engineering principle of fault tolerance, providing two distinct locking mechanisms to guard against failure. Using a certified MS17825-6 nut ensures that the connection is protected by both a friction lock to maintain rigidity and a positive lock to prevent disassembly. It is the definitive choice for joints where the safety of flight is paramount.

The MS17825-6 fastener is a prime example of applying the engineering principle of dissimilar redundancy to a single component. The two locking mechanisms are designed to protect against different failure modes. The nylon insert provides a prevailing torque (friction) lock, which is highly effective at damping high-frequency vibrations and maintaining the bolt's axial preload. The castellated nut and cotter pin provide a positive (mechanical shear) lock. This secondary system is not dependent on friction or preload and is designed to provide a physical barrier to prevent disassembly. This ensures that even in a highly improbable scenario where the primary friction lock is compromised or the joint loses its clamp load, the mechanical lock of the cotter pin will ensure the physical connection remains intact, preventing catastrophic failure.

Note: This is a flight-critical hardware component. Installation must be performed by a certified mechanic in strict accordance with the maintenance manual. The nut should be torqued to the specified value. If the slots in the nut do not align with the hole in the bolt, the nut must be tightened to the next available slot—never loosened. The 250°F temperature limit must be strictly observed. A new cotter pin must be used for every installation.

Documentation

The military standard governing this part is publicly available through the Department of Defense ASSIST database.

• ASSIST Document Summary Page for MS17825: http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=36599