What is the valve-tappet clearance on a running engine with hydraulic lifters?

In an engine equipped with hydraulic lifters, the valve-tappet clearance, often referred to as lash, is effectively maintained at zero when the engine is running. Hydraulic lifters automatically adjust to maintain the correct clearance as the engine operates.

A running engine relies on the hydraulic mechanism within the lifters to compensate for thermal expansion and to ensure that there is adequate contact between the lifter and the valve stem without any gap. Therefore, any perceived clearance is minimized to a negligible amount, making the correct response zero clearance.

While the other answer choices suggest specific measurements for clearance, these figures typically apply to mechanical lifters where adjustments are necessary to account for thermal expansion and wear. Hence, they do not apply in this context where hydraulic lifters are present, as their design inherently eliminates the need for such clearances during operation.