Why is tappet clearance provided in a piston engine?

Providing tappet clearance in a piston engine is essential for accommodating the thermal expansion of the valve gear as the engine warms up. As the engine operates, various components heat up and expand due to increased temperatures. If there were no clearance, this expansion could lead to excessive pressure on the valve openings, potentially causing the valves to remain slightly open even when they should be closed. This can result in loss of compression and efficiency, and may even lead to engine damage.

Additionally, the clearance ensures proper operation of the components, allowing for smoother movement and reducing wear. While factors like manufacturing tolerances and adjustments for valve timing are important considerations in engine design, the key reason for incorporating tappet clearance is to manage the effects of thermal expansion during engine operation. This ensures the engine runs efficiently and maintains performance over various temperature ranges.