Piston rings are designed to seal the gap between the piston and the cylinder wall. If this gap were too small, thermal expansion of the piston could cause it to seize in the cylinder, causing severe damage to the engine. On the other hand, a significant gap would cause insufficient sealing of the piston rings against the cylinder walls, resulting in excessive blow-by (combustion gases entering the crankcase) and less pressure on the piston, reducing the engine’s power output.