Why is it important to have clearance between the cylinder wall and piston?

Having clearance between the cylinder wall and piston is essential because the materials expand at different rates when heated. During engine operation, the piston experiences significant temperature changes due to combustion, which causes it to expand more than the cylinder wall. If there is insufficient clearance, the expanding piston could create excessive friction or even seize against the cylinder wall, leading to engine failure.

This clearance allows for the thermal expansion of the piston while accommodating the fit between the moving parts. If managed correctly, this ensures smooth operation and long-term reliability of the engine. The other options may relate to various aspects of engine function, but they do not directly address the critical need for clearance due to differential thermal expansion.