MS17825-4 - Slotted Castellated Self-Locking Nut

MS17825-4 – 1/4-28 Castellated Self-Locking Nut (Dual-Lock)

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Overview

The MS17825-4 is a high-security, dual-locking nut manufactured to a stringent Military Standard for use in the most flight-critical bolted joints. This fastener is engineered to provide 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 applications where the integrity of a fastened joint must be absolutely guaranteed.

Constructed from high-strength, cadmium-plated steel, this 1/4-28 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: Combines 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 bolt threads, maintaining preload and resisting vibration.

• Castellated for Positive Locking: The slots in the nut are designed to align with a hole in a drilled-shank bolt, allowing a cotter pin to be installed. This creates an infallible physical 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: MS17825-4

• Component Type: Hexagon Castellated Self-Locking Nut

• Governing Specification: MS17825

• Material: Steel, Cadmium Plated

• Locking Element: Nylon Insert

• Thread Specification: 1/4-28 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 MS17825-4 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, rudders, and other control surfaces to hinges and actuators.

• Critical Engine and Pylon Attachments: Used in key structural mounting points for powerplants.

• Landing Gear and Rotor Systems: Securing major structural and pivoting components where absolute reliability is essential.

Why Choose MS17825-4?

For applications requiring this nut, 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 MS17825-4 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 MS17825-4 fastener embodies the engineering principle of dissimilar redundancy. Its two locking systems are designed to counter different failure modes. The prevailing torque from the nylon insert is a friction-based mechanism that is highly effective at damping high-frequency vibrations and maintaining the bolt's axial preload (clamp force). The positive lock from the cotter pin is a mechanical, shear-based mechanism that is not dependent on friction or preload. This secondary system is designed to prevent catastrophic disengagement of the fastener in the unlikely event that clamp load is lost or the primary locking mechanism is compromised. This two-layer, fault-tolerant approach is a hallmark of designing for systems where failure would have the highest consequences.

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 torked 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