MS21042L3 - Textron Aviation Military Standard Self-Locking Extended Washer Nut

MS21042L3 – Textron Aviation Mil-Spec Self-Locking Two-Lug Plate Nut

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Overview

The MS21042L3 is a military-specification, self-locking, two-lug, low-profile plate nut, specified by Textron Aviation for use in critical airframe and powerplant applications. Often called an "anchor nut," this component is designed to be permanently riveted to a structure, creating a fixed, high-strength threaded receptacle in locations where access to the backside of the fastener is impossible. The "L" designation is critical, specifying that the nut is fabricated from A-286 corrosion-resistant steel (CRES), a high-temperature superalloy, making it suitable for use in engine compartments, on firewalls, and in other high-temperature zones.

The nut features an all-metal, prevailing torque locking mechanism, achieved by precisely deforming the upper threads of the basket. This design creates a consistent frictional force on the bolt, providing exceptional resistance to loosening under the most severe vibration. The silver plating acts as a high-temperature, anti-galling agent. For any blind-fastening application in a high-temperature or structurally critical area, the certified MS21042L3 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, such as firewalls and engine nacelles.

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

• 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 with two rivet holes provides a wide footprint to distribute loads into the structure and ensures a secure, permanent installation.

Technical Specifications

• Part Number / Specification: MS21042L3

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

• OEM Designator: Textron Aviation

• Thread Size (-3): #10-32 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

• Textron Aviation & General Aerospace: This is a standard mil-spec plate nut used extensively by Textron Aviation (Cessna, Beechcraft), particularly on their turboprop and jet aircraft, as well as across the entire aerospace industry where a high-temperature, #10-32 anchor nut is required.

Applicable Textron Aircraft

• • 1900 Airliner
  • 1900C Airliner
  • 1900D Airliner
  • Baron Series B55, E55, 58 or G58
  • Beechjet 400/400A
  • Bonanza 36
  • Bonanza F33, G33, F33A or F33C
  • Caravan or Grand Caravan EX
  • Citation CJ1 or CJ1+
  • Citation CJ2 or CJ2+
  • Citation CJ3, CJ3+, CJ3+ Gen 2, CJ3 Gen 3
  • Citation CJ4, CJ4 Gen 2, CJ4 Gen 3
  • Citation Excel, XLS, XLS+, XLS Gen 2
  • Citation III, VI or VII
  • Citation Latitude
  • Citation Longitude
  • Citation M2, M2 Gen 2, M2 Gen 3
  • Citation Mustang
  • Citation Sovereign or Sovereign+
  • Citation V, Ultra, Encore or Encore+
  • Citation X
  • Citation X+
  • King Air 100
  • King Air 350 Fusion
  • King Air 350C FM-36 & On
  • King Air 350C Fusion
  • King Air 350i
  • King Air 360
  • King Air 360C
  • King Air B200, B200T, B200C or B200CT
  • King Air B200GT
  • King Air B200GT Fusion or B200CGT Fusion
  • King Air C90, C90A, C90GT or E90
  • King Air C90GTi
  • King Air C90GTx
  • King Air C90GTx Fusion
  • Premier Model 390
  • SkyCourier
  • Skyhawk
  • Skylane
  • Stationair
  • Super King Air 350
  • Super King Air 350C FM-1 to FM-35


List not comprehensive, may be applicable on other models.

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

• Firewall & Engine Nacelle Installations: The 450°F rating makes it the standard choice for mounting components, fairings, and inspection panels in high-temperature zones.

• Access Panel Attachment: Provides a fixed, reusable thread for securing panels that require routine removal for inspection.

• Component Mounting: Used to attach brackets, clamps, and other hardware to ribs, spars, and bulkheads in blind applications.

Why Choose MS21042L3?

The selection of a plate nut is a critical engineering decision. Using a standard, low-temperature alloy steel nut (e.g., MS21047) in a high-temperature application is a catastrophic risk; the heat will cause the nut to lose its temper and all self-locking capability, leading to fastener failure. The MS21042L3 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 the stainless steel nut from galling and seizing to the bolt, which would necessitate a destructive removal. Sourcing a fully certified, OEM-specified MS21042L3 is the only way to ensure the safety and integrity of a bolted joint in a high-temperature, blind-access location.

The superior high-temperature performance of the MS21042L3 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 MS21042L3 must be installed using two appropriate structural rivets (e.g., AN426A or similar). 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.