MS21043-4 - MS Standards Aerospace Nut

MS21043-4 – MS Standards Aerospace Nut

Available now on the Airparts.aero Marketplace.

Overview

The MS21043-4 is a military-standard, lightweight, all-metal, self-locking hex nut, specifically engineered for aerospace applications requiring a low-profile, high-temperature fastening solution. This nut is designed to provide reliable, vibration-resistant locking action in environments up to 450°F (232°C). Manufactured from high-strength, cadmium-plated steel, the MS21043-4 features a deformed-thread locking mechanism and an integral washer face (ring base), making it a superior choice for firewall-forward applications and other space-constrained, high-temperature structural joints.

Key Features

• All-Metal Locking Design: Features a precision-deformed elliptical thread at the top of the nut. This all-metal locking mechanism creates a powerful spring-like frictional grip on the bolt threads, providing a consistent prevailing torque that is resistant to high temperatures and vibration.
• 450°F Temperature Rating: The absence of a non-metallic (nylon) insert allows this nut to maintain its structural integrity and locking performance at continuous operating temperatures up to 450°F, making it essential for many engine compartment applications.
• Low-Profile, Ring Base Design: Engineered with a reduced height and an integral washer face (ring base). This design provides a low-profile installation with a broad, flat bearing surface that distributes clamping force evenly, all while saving weight and space compared to a standard nut and washer combination.
• High-Strength Steel Construction: Fabricated from a high-quality carbon steel alloy and heat-treated to provide the tensile strength required to properly preload standard AN structural bolts.
• MS Certified Standard: As a certified Military Standard part, it guarantees the material, dimensions, heat treatment, and locking torque performance meet the stringent requirements for use in certified aircraft. It is the superseding part for the legacy AN364-428 nut.

Technical Specifications

• Standard: MS21043 (Nut, Self-Locking, Hexagon, Low Height, Ring Base, 450°F)
• Full Part Number: MS21043-4
• Legacy Part Number: AN364-428
• Component Type: All-Metal Prevailing Torque Self-Locking Nut
• Material: Carbon Steel
• Finish: Cadmium Plated
• Thread Specification: 1/4-28 UNJF-3B
  • Nominal Diameter: 1/4 inch (0.250 inches)
  • Threads Per Inch: 28 (Fine Thread)
• Wrench Size: 7/16 inch
• Maximum Operating Temperature: 450°F (232°C)

Aircraft Compatibility

The MS21043-4 is a universal hardware component used across all categories of aircraft where a low-profile, high-temperature, all-metal locknut is required for a 1/4" bolt.

• Common Applications:
  • Firewall Forward: The primary application is for securing components in the engine compartment, such as engine baffles, accessories, brackets, and exhaust system heat shields, where temperatures would degrade a nylon insert.
  • Brake Systems: Used in close proximity to brake assemblies where high temperatures are generated.
  • Space-Constrained Assemblies: Its low-profile design makes it ideal for installations where clearance is minimal.

Applications

The MS21043-4 is the specified fastener for creating secure joints in challenging environments:

• Heat-Resistant Fastening: Provides a reliable, vibration-proof connection in areas that are too hot for standard AN365 / MS21044 nyloc nuts.
• Lightweight Assemblies: The reduced height and integrated washer base contribute to overall weight savings.

Why Choose MS21043-4?

A standard nylon insert locknut will fail if its 250°F temperature limit is exceeded, resulting in a complete loss of the locking feature. The MS21043-4 provides the next level of thermal protection in a compact form factor. Using a genuine MS-standard nut guarantees that the part has the certified material properties, heat treatment, and precisely engineered locking feature required for a safe and airworthy installation. For space-and-weight-conscious, moderately high-temperature applications, the MS21043-4 is the industry-standard solution.

Order Your MS21043-4 Nuts Now

Secure your aircraft's high-temperature and space-constrained connections with certified, flight-critical hardware. Visit Airparts.aero to order your MS21043-4 all-metal self-locking nuts today.

The locking mechanism of the MS21043-4 is achieved through controlled thread deformation. During manufacturing, the top one or two threads of the nut are precisely deflected inward at opposing points, creating a slightly non-circular shape. As a standard male bolt thread enters this section, it must elastically deflect the deformed nut threads back into a circular shape. This creates a powerful spring-like clamping force, or friction grip, on the flanks of the bolt threads. This purely mechanical interference creates the "prevailing torque" that resists rotation from vibration. Because this mechanism relies solely on the resiliency of the heat-treated steel, it remains effective at temperatures up to 450°F, far beyond the melting point of a nylon polymer insert.

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.