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MS21043-5 – High-Temperature Self-Locking Reduced Hex Nut (5/16-24)
Available now through the AirParts Aero marketplace.
Overview Section
The MS21043-5 is a high-strength, corrosion-resistant (CRES) self-locking nut designed for aerospace environments subject to extreme temperatures. Unlike the MS21042 series, which is typically made of alloy steel, the MS21043 series is manufactured from A286 stainless steel (or equivalent high-temp alloys). This allows the fastener to maintain its structural integrity and locking capabilities in environments up to 1,200°F (649°C).
The nut features a "Reduced Hex" geometry to save weight and allow for installation in narrow clearances. It utilizes a displaced-thread (all-metal) locking mechanism, where the top threads are slightly deformed to create prevailing torque. This prevents the nut from backing off even under intense engine vibration or thermal cycling.
Key Features Section
Extreme Temperature Resistance: Rated for continuous operation up to 1,200°F, making it the standard choice for exhaust manifolds, turbocharger brackets, and jet engine hot sections.
A286 Stainless Steel Construction: Provides superior corrosion resistance and high tensile strength, eliminating the need for cadmium plating in environmentally sensitive or high-heat areas.
All-Metal Self-Locking: The elliptical offset or "crimped" top provides a mechanical lock that does not rely on nylon inserts, which would melt in high-heat applications.
Silver Plated Finish: Typically finished with silver plating to act as a high-temperature anti-seize lubricant, preventing "galling" or thread-locking during installation and removal.
Reduced Hex Design: Features a smaller wrench size than standard AN363 or AN365 nuts, allowing for tighter grouping of fasteners and overall weight reduction.
Non-Magnetic: Due to its stainless steel composition, it is ideal for use near sensitive magnetic compasses or avionics.
Technical Specifications Section
| Specification | Detail |
| Part Number | MS21043-5 |
| Thread Size | 5/16-24 UNJF-3B (Fine Thread) |
| Material | A286 Corrosion-Resistant Steel (CRES) |
| Finish | Silver Plated (per AMS 2410) |
| Locking Type | All-Metal Displaced Thread |
| Max Operating Temp | 1,200°F (649°C) |
| Hex Size (Wrench) | 7/16 inch |
| Tensile Strength | 125,000 PSI Minimum |
Aircraft Compatibility Section
The MS21043-5 is a critical fastener for high-heat and high-corrosion zones:
Turbine Engines (PT6, TFE731, CFM56): Used on exhaust ducts, bleed air valves, and turbine casing flanges.
Piston Engine Exhausts: Standard for securing exhaust stacks to cylinders on Lycoming and Continental engines to prevent nut loss due to heat expansion.
Turbocharger Systems: Vital for wastegate linkages and turbo-mount brackets where standard nuts would lose their temper.
Environmental Control Systems (ECS): Used on high-pressure ducting near the engine's bleed air ports.
Technical Applications & Safety
The MS21043-5 is a prevailing torque nut, meaning it should require effort to turn even before it seats against the washer.4
Safety Warning: Galling Risk with Stainless Fasteners. When using a stainless steel nut (MS21043) on a stainless steel bolt, there is a high risk of "galling" (the threads cold-welding together). The silver plating on the MS21043-5 is designed to prevent this, but the threads should always be inspected for cleanliness. Never use an impact wrench on these nuts; the friction-generated heat can cause immediate seizing. Additionally, verify the prevailing torque by hand before use—if the nut can be spun onto the bolt threads freely by finger, the locking feature is worn out and the nut must be scrapped.
Technical Engineering Insight
The engineering value of the MS21043-5 lies in its Thermal Expansion Compatibility. In a 1,200°F environment, the bolt and the nut expand significantly. Standard alloy steel nuts (like MS21042) have a different coefficient of expansion than the stainless or Inconel bolts used in hot sections, which can lead to the nut loosening as the temperature fluctuates. By using A286 CRES, the MS21043-5 expands at a rate closer to the specialized hardware it secures, maintaining a consistent "clamp load" through the entire thermal cycle of the engine.
Installation & Maintenance Addendum
Thread Integrity: Inspect the silver plating. If the plating is flaking off or heavily worn, the nut should not be reused, as the bare stainless steel will likely seize onto the bolt.
Torque Values: Use the "Dry Torque" values specified in the aircraft maintenance manual (AMM) unless otherwise directed. The silver plating acts as a dry lubricant, so no additional anti-seize is usually required.
Wrenching: Always use 12-point tools for reduced-hex nuts. This provides more contact points and prevents stripping the smaller-than-normal hex corners.
