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MS20073-03-04 – Fine-Thread Hex Head Machine Bolt (Alloy Steel)
Available now on the Airparts.aero Marketplace.
The MS20073-03-04 is a high-strength, precision-engineered aerospace machine bolt manufactured to Military Standard (MS) specifications. Belonging to the MS20073 series, this fastener is designed for structural applications where high tensile strength and fatigue resistance are required. It features a hexagonal head for high-torque wrenching and a non-drilled shank, making it a "standard" structural bolt for airframe, engine, and component assembly.
Constructed from 4130 or 8740 cadmium-plated alloy steel, the MS20073-03-04 is engineered to provide a minimum tensile strength of 125,000 psi. The cadmium plating (typically Type II, Class 2) provides essential lubricity for accurate torque tensioning and offers superior corrosion resistance, particularly in environments where dissimilar metals (such as aluminum airframes) are in contact.
Key Features
High-Tensile Alloy Steel: Heat-treated to a Rockwell hardness of C26-C32, ensuring the bolt can withstand significant structural loads without permanent deformation.
Cadmium Plating (Yellow Chromate): Provides a sacrificial barrier against corrosion and prevents "galling" during installation, which is critical for maintaining consistent preload.
Precision Fine-Thread (UNF): Features a 10-32 UNF-3A thread profile, offering higher stripping strength and better vibration resistance compared to coarse-thread fasteners.
Cold-Upset Head & Rolled Threads: The manufacturing process ensures continuous grain flow, significantly increasing the bolt's resistance to fatigue and "head-popping" under stress.
Standard Hexagonal Drive: Designed for use with standard aerospace sockets and wrenches, allowing for precise torque application in confined spaces.
Technical Specifications
Part Number: MS20073-03-04
Military Standard: MS20073 (Superseding AN3 series in high-strength applications)
Thread Size: #10-32 (0.1900" - 32 Fine Thread)
Bolt Length: 0.250" (1/4 inch) – Measured from under the head to the tip.
Grip Length: 0.062" (Approx.)
Material: Alloy Steel (MIL-S-6049 or MIL-S-6098)
Finish: Cadmium Plated per QQ-P-416, Type II, Class 2
Head Type: Hexagonal (Non-drilled)
Tensile Strength: 125,000 psi Minimum
Aircraft Compatibility
General Aviation: Extensively used in Cessna, Piper, and Beechcraft airframes for fairings, brackets, and fluid line supports.
Commercial Transport: Utilized in Boeing and Airbus secondary structures and interior monument tie-downs.
Experimental / Sport Aviation: The preferred choice for high-stress structural joints in homebuilt aircraft.
Note: The MS20073 series is a direct high-strength replacement for the legacy AN3 series. Always consult your Aircraft Maintenance Manual (AMM) or Structural Repair Manual (SRM) to verify that a "non-drilled" bolt is appropriate for the specific locking method (e.g., lock washer or self-locking nut) used in the assembly.
Applications
The MS20073-03-04 is utilized for Structural Fastening and Component Mounting. Its primary roles include:
Bracket Attachment: Securing avionics trays, hydraulic reservoirs, and electrical junction boxes.
Control System Hardware: Linking non-rotating push-pull rods and bellcranks.
Engine Accessories: Mounting fuel pumps, magnetos, and heat shields where high-vibration resistance is required.
Why Choose MS20073-03-04?
Choosing a genuine MS20073-03-04 ensures Fastener Preload Integrity. In aviation, a bolt's primary job is to act as a "clamp." If a bolt has inconsistent threads or poor plating, the friction during installation will result in an incorrect "Torque-Tension" relationship. This leads to a loose joint that is susceptible to Fatigue Failure. The MS20073-03-04 is manufactured under strict quality controls to ensure that when you apply the specified torque, the bolt reaches its ideal elastic stretch to keep the joint tight for thousands of flight hours.
The MS20073-03-04 functions as a Tension-Loaded Fastener governed by the physics of Hooke’s Law. Within its elastic limit, the bolt acts as a stiff spring. The "Fine-Thread" (UNF) profile provides a smaller Helix Angle ($\beta$), which translates to a higher mechanical advantage during tightening and a reduced tendency for the bolt to "back-out" under axial vibration. The Cadmium plating is critical not just for corrosion, but for defining the Coefficient of Friction ($\mu$). In aerospace maintenance, this ensures that the torque applied to the head is efficiently converted into Clamping Force ($F_c$), rather than being lost to frictional heat in the threads.
Note: This is a structural fastener. Never reuse a bolt that shows signs of "necking," corrosion, or thread damage. Always use a calibrated torque wrench for installation. Because this bolt is "non-drilled," it must be secured using a secondary locking device such as an MS21042 self-locking nut or an AN935 lock washer, as specified by the engineering drawing.
