MS24665-351 – Military Standard 5/32" x 1-1/2" Steel Cotter Pin
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The MS24665-351 is a flight-critical, positive locking device manufactured in strict accordance with Military Standard MS24665. Commonly known as a split pin, its sole and essential function is to secure castellated nuts on drilled-shank bolts, studs, and clevis pins, physically preventing them from loosening due to vibration. This simple, reliable, and visually inspectable component is one of the most fundamental safety devices used in aircraft construction and maintenance, ensuring the integrity of thousands of critical bolted joints throughout the airframe and engine.
This cotter pin is manufactured from a specific low-carbon steel wire that is engineered for ductility, allowing its tines to be bent securely without fracturing. It is then cadmium plated for corrosion resistance. As a mandatory single-use part, the MS24665-351 is designed to be plastically deformed during installation, providing a near-foolproof method of fastener retention.
Key Features
Military Standard (MS) Certified: Guarantees that the pin meets the stringent material, dimensional, and quality control requirements for certified aircraft use.
Provides Positive Mechanical Lock: When properly installed in a castellated nut, it creates a physical obstruction that makes it impossible for the nut to back off, ensuring the joint remains torqued.
Ductile Carbon Steel Construction: Made from a soft, cold-drawn steel alloy that is designed to be bent without fracturing, providing a reliable and secure installation.
Cadmium Plated for Corrosion Resistance: Coated with a sacrificial layer of cadmium per specification QQ-P-416, providing excellent corrosion protection.
Single-Use for Maximum Safety and Reliability: Engineered to be replaced every time a fastener is removed to eliminate the risk of failure from metal fatigue.
Technical Specifications
Part Number: MS24665-351
Component Type: Split Cotter Pin
Governing Specification: MS24665
Material: Low-Carbon Steel, Cold Drawn.
Finish: Cadmium Plated per QQ-P-416, Type II, Class 3.
Dimensions:
Nominal Diameter: 5/32 inch (0.156 in)
Length (Under head to end of short leg): 1-1/2 inches (1.500 in)
Aircraft Compatibility
The MS24665-351 is a standard hardware component used in countless applications across all categories of certified aircraft from all manufacturers, including Textron, Piper, Boeing, Airbus, and others. It is specified wherever a castellated nut (e.g., AN310, AN320) is used to secure a fastener in a critical application.
Crucial Note: The correct diameter and length of the cotter pin for a specific application must be verified against the aircraft's Illustrated Parts Catalog (IPC) or maintenance manual.
Applications
The MS24665-351 is used exclusively as a locking device for nuts and other fasteners. It is designed to take a shear load only and must never be used as a structural pin. Common applications include securing the castellated nuts on:
Flight Control Linkages and Clevis Pins
Landing Gear Components, including Wheel Axle Nuts and Torque Links
Engine Mounting Hardware
Structural Airframe Bolts and Attachments
Why Choose MS24665-351?
Choosing a genuine MS-specification MS24665-351 cotter pin is a critical decision for flight safety. Re-using a cotter pin is exceptionally dangerous, as the process of straightening and re-bending the tines severely fatigues the metal, making it prone to fracturing under normal operational vibration. The failure of a cotter pin can allow a critical nut—such as a wheel axle nut or a flight control bolt nut—to back off, leading to a catastrophic mechanical failure. Using a non-certified commercial-grade pin introduces the risk of an improper material that may be too brittle to install correctly or too weak to perform its function. The MS standard is the only guarantee of the certified material and quality required for this essential safety device.
The mandatory single-use nature of the MS24665-351 is rooted in the material science of work-hardening (or strain-hardening). The low-carbon steel of the pin is supplied in a soft, annealed state, which provides the high ductility required to bend the tines without fracturing. This bending action, however, is a form of plastic deformation, a process that creates and tangles a high density of dislocations within the metal's crystal lattice. This change in the microstructure makes the material significantly harder, stronger, and less ductile (more brittle) in the bent area. When a used pin is straightened for removal, this already work-hardened area is subjected to a reverse bending stress, further fatiguing the metal. Attempting to re-bend this now-brittle tine during a second installation creates extreme stress concentrations at the bend radius. This makes the tine highly susceptible to fracture, either during the installation itself or, more dangerously, later from normal in-flight vibration, which would render the locking mechanism useless.
Note: This is a flight-critical, single-use component. A new cotter pin MUST be used for every installation. Re-using a cotter pin can lead to fatigue failure of the pin, loss of the nut, and catastrophic failure of the joint. The pin should be installed so that the head is fully seated, and the tines should be bent in accordance with the procedures and options detailed in standard maintenance practices like FAA Advisory Circular AC 43.13-1B.
Documentation
The definitive technical requirements for this component are detailed in Military Standard MS24665. These proprietary documents must be sourced directly from the relevant standards organizations for complete engineering specifications.