MS21042-4 – Two-Lug Anchor Nut, 1/4-28, with Nylon Lock (250°F)
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Overview
The MS21042-4 is a high-quality, 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 assembly, designed to provide a permanent, load-bearing 1/4-28 threaded attachment point in blind locations where access to the backside of a structure is impossible. It features a two-lug base for secure rivet installation and is fabricated from durable, cadmium-plated steel.
The defining characteristic of the standard MS21042-4 (with a blank material code) is its self-locking mechanism: a captive nylon insert. This provides a consistent and reliable prevailing torque to resist loosening from vibration in general airframe applications. This component is rated for a maximum service temperature of 250°F (121°C), distinguishing it from its all-metal, high-temperature counterparts (MS21042L4).
Key Features
Two-Lug Plate Design: The integral base with two lugs is designed to be securely riveted to the airframe, providing exceptional resistance to torque-out and pull-out forces.
Nylon Insert Self-Locking: Features a captive nylon collar that grips the bolt threads to create a consistent prevailing torque, providing excellent resistance to loosening from vibration.
250°F Temperature Rating: Certified for use in a wide range of general airframe and interior applications where temperatures will not exceed 250°F (121°C).
Cadmium-Plated Steel: The nut body is fabricated from high-strength steel and plated with cadmium per QQ-P-416 for durability and superior corrosion resistance.
Enables Blind Assembly: Allows for the secure attachment of components in closed-box structures, greatly simplifying airframe assembly, maintenance, and repair.
Fixed Nut Element: The threaded portion is integral to the nut body, providing a rigid, non-floating attachment point for applications requiring precise alignment.
Technical Specifications
Part Number: MS21042-4
Component Type: Self-Locking Plate Nut / Anchor Nut (Two-Lug, Low Height, Fixed)
Governing Specification: MS21042 / NASM21042
Material: Steel, Cadmium Plated per QQ-P-416
Locking Element: Nylon Insert (or equivalent non-metallic, 250°F rated)
Thread Specification: 1/4-28 UNF-3B
Maximum Operating Temperature: 250°F (121°C)
Installation Method: Attaches with two 3/32" rivets (typically AN426AD3 countersunk type).
Aircraft Compatibility
The MS21042-4 is a universal airframe construction component used on virtually all classes of certified aircraft where a 250°F rated anchor nut is required. 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 fairings in general airframe locations.
Securing floor panels, interior components, upholstery, and cabin systems.
Mounting avionics racks, equipment trays, and instrument panels.
Attaching brackets and clips to ribs, stringers, and skin in areas not exposed to high temperatures (i.e., not firewall forward).
Why Choose MS21042-4?
Using the correct temperature-rated hardware is critical for flight safety. The MS21042-4 is the industry standard for reliable, self-locking anchor nuts in ambient temperature locations. Its nylon locking element provides excellent reusability and consistent torque performance. Using this 250°F rated nut in a high-temperature zone would lead to the failure of the locking element. Conversely, using a more expensive all-metal nut where it is not required is inefficient. The certified MS21042-4 ensures you have the correct, specified part for the vast majority of standard airframe assemblies.
The self-locking capability of the MS21042-4 is achieved through the principle of prevailing torque, generated by its integrated nylon locking collar. The internal diameter of the nylon element is deliberately smaller than the major diameter of the mating 1/4-28 bolt. As the bolt is threaded into the nut, it forces its way through the collar, causing the resilient nylon to be elastically deformed. This deformation creates a uniform, 360-degree radial compressive force, or grip, on the bolt threads. This sustained pressure generates a consistent frictional torque that resists rotation and is independent of the clamp load on the joint. This mechanism is highly effective at damping vibration and preventing the nut from backing off, providing a secure and reliable joint in the vast majority of airframe applications.
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 data. The 250°F temperature limitation must be strictly observed. Correct rivet selection, hole preparation, and rivet installation technique are critical to the 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