Define a shim operation and its context in overhaul.

• A. Installing shims to seal external surfaces.
• B. Using shims to adjust fuel flow timing.
• C. Replacing worn wear rings only.
• D. Inserting shims to adjust radial clearances between rotating and stationary parts to meet specified tolerance.

Answer: (D)

Shimming is the process of inserting thin pieces of material between mating rotating and stationary parts to set the exact radial clearance. In an overhaul, keeping the rotor-to-stator gap within the specified tolerance is crucial for preventing rubbing, controlling oil leakage, and ensuring proper rotor dynamics and bearing loading. When wear or machining alters the clearance, adding shims of precise thickness restores the intended spacing and keeps the assembly operating as designed. Shims are used only where the manufacturer specifies, and their placement must follow the approved procedure to ensure even clearance and proper alignment. The other activities mentioned—sealing external surfaces, adjusting fuel flow timing, or replacing wear rings—do not address radial clearance adjustment with shims, so they don’t describe a shim operation.