73383 - Lycoming Lock Plate

73383 – Lycoming™ Crankshaft Gear Bolt Lock Plate

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The Lycoming 73383 is a flight-critical, genuine OEM lock plate that serves one of the most vital functions in a direct-drive Lycoming engine: providing a positive mechanical lock for the crankshaft gear retaining bolt. This single, deceptively simple component is indispensable for safety, as it physically prevents the loosening of the bolt that secures the timing gear to the nose of the crankshaft. The integrity of this joint is paramount, as the crankshaft gear drives the camshaft (for valve timing) and the accessory case gears (for ignition timing).

Manufactured from a specific malleable steel alloy, this plate is designed with tabs that are keyed to the crankshaft gear. After the main retaining bolt (P/N STD-2209) is torqued to its specified value, the plate's outer tabs are physically bent up against the flats of the bolt head. This creates a mechanical interference that makes it physically impossible for the bolt to rotate and loosen due to engine vibration and harmonics, thereby securing the engine's critical valve and ignition timing.

Key Features

• Genuine Lycoming OEM Part: The only part guaranteed to meet the exact material, ductility, and dimensional specifications required for this critical application.

• Provides Positive Mechanical Locking: A superior locking method that creates a physical stop, preventing fastener rotation far more reliably than friction-based devices.

• Prevents Catastrophic Timing Failure: Secures the single most important bolt responsible for maintaining the engine's mechanical timing.

• Ductile Steel for Secure Tab Bending: Made from a specific grade of steel that is soft enough to be bent into place without fracturing, yet strong enough to resist shearing under operational stresses.

• Single-Use Component for Maximum Safety: Engineered to be replaced every time it is used to eliminate the risk of failure from metal fatigue.

Technical Specifications

• Part Number: 73383

• Component Type: Crankshaft Gear Bolt Lock Plate

• OEM Manufacturer: Lycoming Engines

• Material: Malleable Steel Alloy

• Finish: Cadmium Plated for corrosion resistance.

• Application: Provides a positive lock for the crankshaft gear retaining bolt (P/N STD-2209) on direct-drive Lycoming engines.

Engine Compatibility

The 73383 lock plate is one of the most widely used parts in the Lycoming ecosystem and is specified for nearly all direct-drive Lycoming engine families, including but not limited to:

• O-235, O-320, IO-320, AEIO-320 Series

• O-360, IO-360, AEIO-360, TIO-360 Series

• O-540, IO-540, AEIO-540, TIO-540, LTIO-540 Series

• IO-580 Series

• IO-720 Series

Crucial Note: The specific engine model and serial number must be checked against the latest revision of the official Lycoming Parts Catalog to ensure correct part number applicability. The engine parts catalog is the final authority.

Fits in 

Applications

This lock plate is a mandatory replacement part during any maintenance that requires the removal of the crankshaft gear retaining bolt. This includes:

• Complete Engine Overhaul

• Crankcase Separation and Repair

• Camshaft or Crankshaft Replacement

Why Choose 73383?

Choosing a genuine Lycoming 73383 lock plate is a non-negotiable matter of flight safety. The failure of this single component would lead to the loosening of the crankshaft gear bolt, resulting in an instantaneous and catastrophic loss of engine power. Re-using an old lock plate is exceptionally dangerous, as the metal becomes work-hardened and fatigued after being bent once, making the tabs prone to fracturing under vibration. Using a non-genuine part introduces risks of improper fit or incorrect material that may not have the required ductility. The negligible cost of a new, genuine Lycoming 73383 is the ultimate insurance for the mechanical integrity of the entire engine.

The engineering principle embodied by the 73383 is positive locking, a method mechanically superior to frictional locking for ultimate-security applications. Frictional locking methods (e.g., lock washers, prevailing torque nuts) and even restraining devices (e.g., safety wire, cotter pins) merely add significant resistance to the loosening of a fastener. Under extreme vibration, thermal cycling, or harmonic stress, these methods can theoretically be overcome. Positive locking, by contrast, creates a direct physical interference between the fastener and a stationary component. The 73383 plate is keyed to the stationary crankshaft gear, and its tabs are bent to physically block the flats of the bolt head. This design does not rely on friction or tension to prevent rotation; it creates a solid mechanical stop. The only possible failure mode is the physical shearing of the bent tab. This is precisely why the part is made from a specific ductile steel and, more importantly, why it is a mandatory single-use component. Bending the tab induces work-hardening and fatigue in the metal, and re-bending a previously used tab would create a stress riser, making it highly susceptible to fracture.

Note: This is a flight-critical, single-use component. This lock plate MUST be replaced with a new one every time the crankshaft gear bolt is removed. Re-using a lock plate that has been previously bent will lead to metal fatigue and potential fracture of the locking tab, which can result in a catastrophic, uncontained engine failure. Installation must be performed by a certified A&P mechanic in strict accordance with the applicable Lycoming overhaul manual.

Documentation

The definitive part number applicability and installation procedures are detailed in the applicable Lycoming engine maintenance, overhaul, and parts catalogs for the specific engine model.