MS21042L5 - Self-Locking Extended Washer Hex Nut

MS21042L5 – Mil-Spec Self-Locking Two-Lug Plate Nut

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Overview

The MS21042L5 is a military-specification, high-temperature, self-locking, two-lug plate nut. While colloquially described as a hex nut with an extended washer, its correct technical term is an "anchor nut" or "plate nut." It is engineered to be permanently riveted to a structure, creating a fixed, high-strength threaded receptacle for a 5/16-24 bolt in locations where it is impossible to access and hold a conventional nut. The "L" designation in the part number is a critical designator, specifying that the nut is fabricated from A-286 corrosion-resistant steel (CRES), a high-temperature superalloy, making it the required component for use in engine compartments, on firewalls, and within hot nacelle structures.

This nut features a robust, all-metal, prevailing torque locking mechanism, achieved by precisely deforming the upper threads of the nut basket. This design provides exceptional resistance to loosening under the most severe vibration and thermal cycling. The nut and threads are silver-plated to prevent galling at high temperatures. For any 5/16-24 blind-fastening application in a high-temperature or structurally critical area, the certified MS21042L5 is the definitive engineering solution.

Key Features

• Mil-Spec Certified: Manufactured and tested in strict accordance with MS21042 (NASM21042), guaranteeing material, performance, and dimensional integrity.

• A-286 CRES Superalloy Construction: The "L" designation specifies A-286, a precipitation-hardenable superalloy that provides exceptional strength and corrosion resistance at elevated temperatures.

• 450°F Service Rating: Certified for continuous service in high-temperature environments where standard steel or lower-grade CRES would fail.

• All-Metal Prevailing Torque Lock: An integrally formed locking feature provides consistent, reliable self-locking performance that is not degraded by temperature, age, or fluid contamination.

• Silver Plating for Anti-Galling: Silver plated per aerospace specifications to act as a high-temperature solid lubricant, preventing galling and seizing between the nut and bolt threads.

• Two-Lug Base for Riveting: The base plate, or "extended washer," distributes the fastener's clamp load into the structure and provides two holes for secure, permanent rivet attachment.

Technical Specifications

• Part Number / Specification: MS21042L5

• Component Type: Nut, Self-Locking, Plate, Two Lug, Low Height, Fixed

• Thread Size (-5): 5/16-24 UNF-3B

• Material / Temp Code (L): A-286 Corrosion-Resistant Steel (CRES), rated for 450°F (232°C).

• Finish: Silver Plated per QQ-S-365.

• Governing Specification: MS21042 / NASM21042.

Aircraft Compatibility

• Universal Aerospace Application: This is a standard mil-spec plate nut used extensively across the entire aerospace industry on high-performance aircraft where a high-temperature, 5/16-24 anchor nut is required. It is found on aircraft from:

  • Textron Aviation (Cessna, Beechcraft), Boeing, Lockheed Martin, Bombardier, Gulfstream, and others.

Note: The correct plate nut for any application must be verified against the aircraft's specific Illustrated Parts Catalog (IPC) or Structural Repair Manual (SRM).

Applications

The combination of a larger 5/16" thread size and high-temperature A-286 material makes this nut ideal for more structurally demanding applications:

• Engine Pylons, Nacelles, and Firewalls: Securing structural components, heat shields, and critical access panels in the hottest parts of the powerplant installation.

• High-Strength Component Mounting: Attaching heavier components, brackets, and equipment to bulkheads and ribs in blind applications.

• Structural Access Panels: Securing larger panels that may carry significant aerodynamic or structural loads.

Why Choose MS21042L5?

The selection of a plate nut in a high-temperature area is a critical engineering decision. Using a standard, low-temperature alloy steel nut (e.g., MS21047) in a location requiring an "L" designated nut is a catastrophic risk; the heat will cause the nut to lose its temper and all self-locking capability, leading to fastener failure and the potential loss of a critical component. The MS21042L5 is specifically engineered with A-286 superalloy to maintain its strength and locking force at 450°F. The silver plating is equally critical to prevent galling. Sourcing a fully certified MS21042L5 is the only way to ensure the safety and integrity of a critical bolted joint in a high-temperature, blind-access location.

The superior high-temperature performance of the MS21042L5 is a direct result of the metallurgy of its A-286 superalloy construction. A-286 is an austenitic, precipitation-hardenable iron-nickel-chromium alloy. Its strength is derived from a specific heat treatment cycle that causes the formation of microscopic, ordered intermetallic precipitates of the gamma prime phase (γ′), primarily Ni3​(Al,Ti). These finely dispersed precipitates act as powerful obstacles to dislocation movement within the alloy's crystal lattice, providing exceptional tensile strength and creep resistance at elevated temperatures. Simultaneously, the silver plating (per QQ-S-365) acts as a high-temperature solid lubricant. Under the extreme pressure of torquing, the soft silver shears and coats the thread surfaces, preventing the microscopic cold-welding (galling) that would otherwise occur between the two stainless alloy surfaces, ensuring the fastener can be properly torqued and later removed without damage.

Note: The MS21042L5 must be installed using two appropriate structural rivets (e.g., AN426AD4-x). These are prevailing torque nuts and have a limited reusability life; they should be checked for minimum prevailing torque before reuse, in accordance with the aircraft maintenance manual. Installation must be performed by a certified A&P mechanic.

Documentation

• NASM21042 (Standard): The current standard for these plate nuts is a controlled document. However, the legacy MS21042 specification sheet, which defines the core dimensional, material, and performance requirements, is often available through online mil-spec libraries.

  
  

  
  

• QQ-S-365 (Plating Specification): This standard for silver plating is available through various online specification databases.

Note: This is a flight-critical component. It must be tightened using a calibrated torque wrench to the value specified in the applicable maintenance manual. While reusable, the nut must be discarded if its prevailing torque falls below the minimum specified value, which can be checked with a torque wrench before the nut is seated.Alternate Part Numbers: 010210-006, 10121717, 2110063, 2628856, 5310PL0141537, 984050, CIPRD-MS21042-L5, FN12M054, H14-5, MS21042-L5, NAS1291-5, NAS679A5, RM52LH3324-054, T6S524, T98S524

Product Overview

A nut having 6 flat sides and an integral washer that extends beyond the flats or periphery. An integral feature is incorporated for locking on the threads of a mating member.

Item Specification