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NAS1726-4E – Aerospace Self-Locking Captive Washer Nut
Available now on the Airparts.aero Marketplace.
The NAS1726-4E is a high-performance, self-locking hex nut featuring an integrated, free-spinning captive washer. Manufactured to the National Aerospace Standard (NAS), this fastener is engineered for applications where high vibration resistance, consistent clamping force, and protection of the mating surface are paramount. The "captive" design ensures that the washer remains permanently attached to the nut body, eliminating the risk of Foreign Object Debris (FOD) caused by dropped washers and significantly reducing installation time in complex airframe assemblies.
The "E" suffix designates that this nut is fabricated from A-286 Corrosion Resistant Steel (CRES), a precipitation-hardened austenitic alloy designed for high-strength applications in environments subject to heat and corrosion. The self-locking mechanism is achieved through an elliptical deformation of the upper threads (prevailing torque), which provides a positive lock that does not require safety wire or cotter pins. For the aerospace technician, the NAS1726-4E represents a reliable, one-piece fastening solution that maintains structural integrity under the cyclic loading conditions of modern turbine engines and airframes.
Key Features
Integrated Captive Washer: Provides a permanent, free-spinning bearing surface that prevents the nut from marring the aircraft structure during torquing.
All-Metal Self-Locking Design: Employs prevailing torque through an elliptically offset locking element, ensuring the nut remains secure despite extreme high-frequency vibration.
A-286 CRES Construction: Offers exceptional strength and oxidation resistance at elevated temperatures, making it ideal for engine nacelles and exhaust-adjacent structures.
Silver Plated Finish: Typically features a silver plating (per AMS 2410) to provide high-temperature lubrication and prevent galling of the stainless steel threads.
FOD Prevention: The one-piece assembly eliminates the danger of loose washers falling into critical areas during maintenance or assembly.
Technical Specifications
Part Number: NAS1726-4E
Thread Size: #10-32 UNJF-3B (Fine Thread)
Material: A-286 Corrosion Resistant Steel (ASME SA638)
Finish: Silver Plated (AMS 2410) or Passivated (dependent on mod-status)
Locking Feature: Elliptical offset (Prevailing Torque)
Nut Style: Hexagon, Captive Washer, Self-Locking
Temperature Rating: Up to 800°F (427°C) for silver-plated variants
Governing Specifications: NAS1726, MIL-DTL-25027
Certification: FAA-PMA / OEM Standard Hardware
Aircraft Compatibility
Boeing 737, 757, 767, and 777 Series
Airbus A320 and A330 Family
Gulfstream GIV, GV, and G650
Bombardier Challenger and Global Express
Note: This fastener is widely used in engine pylons, avionics racks, and internal structural brackets. Always consult the Illustrated Parts Catalog (IPC) for specific aircraft effectivity.
Applications
Engine Pylon Components: Securing heat shields and brackets in high-temperature zones.
Avionics Rack Mounting: Providing a vibration-resistant, non-marring interface for electronic equipment.
Structural Brackets: Joining aluminum or composite members where a separate washer would be difficult to install.
Why Choose NAS1726-4E?
The NAS1726-4E solves several engineering challenges in a single component. By utilizing A-286 steel, it provides a high Strength-to-Weight Ratio and maintains its Modulus of Elasticity at temperatures where standard carbon steel would lose its temper. The integrated washer ensures that the Nut Factor (K) remains stable by providing a consistent, factory-controlled bearing surface, which is critical for achieving the precise Preload required in structural joints. Choosing this NAS-certified hardware ensures that your maintenance meets the rigorous standards required for Part 25 and Part 23 aircraft.
The NAS1726-4E functions on the principle of Prevailing Torque Elastic Deformation. The upper portion of the nut is precision-engineered with an elliptical "squeeze." When the bolt enters this section, the nut must elastically deform to accommodate the circular bolt threads. This creates a high frictional force—independent of any axial load—that prevents the fastener from rotating under Stochastic Vibration.
From a materials science perspective, the A-286 (Iron-Nickel-Chromium) alloy is precipitation-strengthened, providing high creep-rupture strength. This is essential for maintaining Joint Integrity in thermal-cycling environments. The silver plating acts as a Solid-Film Lubricant, preventing the microscopic "welding" known as Galling or Seizure, which is common when stainless steel threads are under high tension. Furthermore, the captive washer design ensures Uniform Clamping Stress distribution, preventing localized Galvanic Corrosion by providing a consistent interface between the high-strength nut and the airframe structure.
Note: The NAS1726-4E is a structural fastener. It is a self-locking nut intended for limited reuse. If the nut can be turned onto the bolt threads by hand past the locking element, the prevailing torque is insufficient, and the nut must be discarded. Silver-plated nuts must be handled with care to avoid contamination. Installation must be performed by a certified A&P mechanic in accordance with AC 43.13-1B and the aircraft's Structural Repair Manual (SRM).
