When should valve tappet clearances be checked for optimal performance?

Checking valve tappet clearances when the engine is cold is crucial for optimal performance because it ensures that the engine components are in their relaxed state, allowing for the most accurate measurement of the clearances. When the engine is cold, the metal parts contract, and the gaps between the valve and tappet are at their minimum, reflecting the manufacturer's specifications for clearances.

If adjustments are made when the engine is hot or running, the metal expands due to heat, which could lead to incorrect measurements because the tappets may not have the proper clearance needed when the engine is at its normal operating temperature. This is particularly important for ensuring that the valves function effectively, thereby maintaining engine performance and preventing potential damage from incorrect valve operation.