Radial impact points on the piston top land

Piston noise that is clearly audible externally is caused by the piston head alternating striking the cylinder running surface.

Depending on the cause, the piston top land strikes either in the tilting direction or in the oval plane (piston pin direction) against the cylinder wall.

• Excessive installation clearances and hence poor guidance of the piston due to excessively large bored or honed cylinders.
• The installation direction was not observed for pistons with a piston pin axis offset.
• Tight connection of the piston pin bed: as a result, the piston head strikes against the cylinder running surface around the tilting axis of the piston pin. Reasons for this are:
  • Insufficient clearance in the connecting rod eye or in the piston pin bore.
  • Excessively narrow fit of the piston pin in the connecting rod bush (shrink-fit connecting rod). If the fit of the piston pin is too tight in the connecting rod eye when the piston pin is shrunk in, the connecting rod eye is deformed in the direction of the narrowest wall thickness. The connecting rod eye and the piston pin take on an oval form in the process. This results in restricted clearance between the piston pin and the piston.
  • Seized piston pin.

• If the connecting rod is misaligned, particularly in the case of a twisted connecting rod or excessive connecting rod bearing clearances, the piston head moves in a pendulum motion in the piston pin direction and strikes against the cylinder.
• Connecting rod misalignment (distortion/twisting): this results in alternating axial thrust in the piston pin, as a result of which the piston pin strikes alternately against the circlips.