MS21042L4 - Extended Self-Locking Washer Nut

MS21042L4 – Two-Lug Fixed Anchor Nut, 1/4-28, Steel (450°F)

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Overview

The MS21042L4 is a high-strength, all-metal, self-locking anchor nut, commonly known as a plate nut, manufactured to a precise Military Standard. This fastener is a fundamental component of modern airframe construction, specifically designed to provide a permanent and reliable threaded attachment point in "blind" locations where the backside of the structure is inaccessible. The MS21042L4 is fabricated from cadmium-plated steel and features an all-metal locking mechanism, giving it a high service temperature rating of 450°F (232°C).

Featuring a two-lug base, this anchor nut is designed to be securely riveted to a structural member, such as a rib, spar, or skin panel, before the final assembly. Once installed, it allows a 1/4-28 bolt to be easily inserted and torqued from the accessible side. This component's robust, fixed design and reliable self-locking feature make it essential for attaching inspection panels, fairings, and countless other components throughout the aircraft.

Key Features

• Two-Lug Plate Design: The integral base with two lugs allows the nut to be securely riveted to the airframe structure, providing exceptional resistance to torque and pull-out forces.

• Enables Blind Assembly: Provides a permanent, load-bearing threaded point in closed-box structures or other areas where a conventional nut cannot be installed.

• All-Metal Locking Mechanism: The upper threads of the nut barrel are mechanically deformed to provide a strong prevailing torque that resists loosening from vibration, without the use of nylon inserts.

• High-Temperature Capability: The 'L' designation signifies a cadmium-plated steel construction rated for continuous operation in temperatures up to 450°F (232°C).

• Fixed Nut Element: The threaded portion is integral to the nut body, providing a rigid, non-floating attachment point for applications requiring precise alignment.

• Cadmium-Plated Steel: Offers a combination of high strength and excellent corrosion resistance for long-term durability.

Technical Specifications

• Part Number: MS21042L4

• Component Type: Self-Locking Plate Nut / Anchor Nut (Two-Lug, Low Height, Fixed)

• Governing Specification: MS21042 / NASM21042

• Material: Steel (Carbon or Alloy)

• Finish: Cadmium Plate per QQ-P-416

• Thread Specification: 1/4-28 UNF-3B

• Maximum Operating Temperature: 450°F (232°C)

• Locking Principle: Prevailing torque via deformed threads.

• Installation Method: Attaches with two 3/32" rivets (typically AN426AD3 countersunk type).

Aircraft Compatibility

The MS21042L4 is a fundamental airframe construction component used on virtually all classes of certified aircraft, from light general aviation models to the largest military and commercial transport jets. It is a staple component in the manufacturing of aircraft from OEMs like:

• Textron Aviation (Cessna, Beechcraft, Hawker)

• Boeing

• Airbus

• Bombardier

• Embraer

• And all other major airframe manufacturers.

Note: Verification of applicability is the responsibility of the installer. Always consult the aircraft's Structural Repair Manual (SRM) or Illustrated Parts Catalog (IPC) for the correct hardware callout.

Applications

• Attaching inspection panels, access doors, and service panels to the airframe.

• Mounting of fairings, fillets, and aerodynamic surfaces.

• Securing floor panels, interior components, and upholstery.

• Mounting avionics racks, equipment trays, and instrument panels.

• Attaching brackets and clips to primary structures like ribs, stringers, and spars.

Why Choose MS21042L4?

Anchor nuts are a cornerstone of efficient, strong, and maintainable airframe design. The MS21042L4 provides a high-strength, heat-resistant, and vibration-proof threaded attachment that is essential for modern aircraft assembly. Using a certified MS-standard part guarantees the material strength, dimensional accuracy, and locking torque performance required for a safe and airworthy installation. The use of non-certified hardware would compromise the structural integrity of the connection point, risking component failure.

The MS21042L4 anchor nut is a key enabler of modern semi-monocoque airframe construction, which relies on assembling stressed-skin panels over a framework of ribs, spars, and stringers. This design creates numerous closed-box structures where access is limited to one side. The anchor nut solves this "blind fastening" problem by becoming an integral part of the substructure. It is riveted in place during sub-assembly, and the rivets transfer the tensile, shear, and torque loads from the subsequent bolt installation directly into the airframe. The all-metal, prevailing torque locking feature is achieved by deforming the nut's threaded barrel into a slight oval shape. As the bolt passes through, it must elastically re-form the barrel into a circle, creating a powerful spring-like grip on the threads that provides exceptional resistance to loosening under the complex vibratory loads experienced during flight.

Note: This is a structural hardware component. Its installation must be performed by a certified mechanic in strict accordance with the aircraft's Structural Repair Manual (SRM) or other approved engineering data. Proper hole preparation, rivet selection (material and size), and rivet installation technique are critical to the ultimate strength and airworthiness of the joint. The self-locking feature is reusable for a limited number of bolt installation cycles.

Documentation

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

• ASSIST Document Summary Page for MS21042: http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=36974

Item Specification

 DESCRIPTION

Military Standard Locking Extended Washer Nut Self

• Self-locking, hexagon style
• Dry film lubricant
• Material: Steel
• Finish: Cadmium overall and chromate overall

Applicable Textron Aircraft

• 1900 Airliner
• 1900C Airliner
• 1900D Airliner
• Baron Series B55, E55, 58 or G58
• Beechjet 400/400A
• Caravan or Grand Caravan EX
• Citation CJ1 or CJ1+
• Citation CJ2 or CJ2+
• Citation CJ3 or CJ3+
• Citation CJ4
• Citation III, VI or VII
• Citation Latitude
• Citation Longitude
• Citation M2
• Citation Mustang
• Citation Sovereign or Sovereign+
• Citation V, Ultra, Encore or Encore+
• Citation X
• Citation X+
• Citation XLS, XLS+ or XLS Gen2
• King Air 100
• King Air 350 or 350C FM-1 to FM-35
• King Air 360, 350i or 350C FM-36 & on
• King Air B200, B200T, B200C or B200CT
• King Air B200GT or B200CGT
• King Air C90, C90A, C90GT or E90
• King Air C90GTi Series
• Premier Model 390
• SkyCourier
• Skyhawk
• Skylane
• Stationair

List not comprehensive, may be applicable on other models.

Note: Verify compatibility with your aircraft’s maintenance manual and consult with a certified aviation technician for proper installation