Piston Slap

The noise often fades as the engine warms up and the piston expands. It can come from worn pistons or cylinders or from the design of some engines. It sounds hollow and is heard at idle and light load. Thicker oil can reduce the noise slightly but does not fix the cause. Measuring piston-to-cylinder clearance during an engine teardown confirms the problem.

Extra clearance lets the piston tilt at the change of direction, so its skirt hits the cylinder wall.

Mild cold piston slap is often harmless, but noise that stays when warm needs investigating. A mechanic's stethoscope helps tell piston slap apart from lifter or bearing noise.