Piston running noises can be caused by a wide variety of influences during engine operation.
- Tilting of the pistons due to excessive running clearance:
The piston can tilt if the dimensions of the cylinder bore are too large or as a result of wear or skirt collapse, stimulated by the pendulum motion of the connecting rod and the change of bearing surfaces of the piston in the cylinder. The piston head hits hard against the cylinder running surface as a result.
- The correct installation direction of the piston was ignored:
In order to smooth out the change of the bearing surfaces of the piston before TDC and before the power stroke, the piston pin axis is offset by some millimetres towards the piston pressure side. If the piston is inserted the wrong way round into the cylinder (i.e. rotated by 180°) and therefore the piston pin axis is offset to the wrong side, then the piston changes bearing surface at the wrong time. The piston tilting is then much heavier and much noisier.
- Tilting of the piston caused by a stiff connecting rod bearing:
The clearance between the piston pin and the connecting rod bush can either be too small by design, or it may have been eliminated by jamming or distortion. This can happen as a result of connecting rod misalignment (bending and twisting).
- Piston striking in the direction of the piston pin:
Any lateral striking of the cylinder bore by the piston mostly stems from misalignment of the connecting rod (bending or in particular twisting): the piston performs a pendulum movement during its upward/downward stroke in the longitudinal axis of the engine, as a result of which the piston strikes in an alternating sequence against the cylinder. Asymmetrical connecting rods or eccentric support for the piston by the connecting rod have the same effect.
- Piston pin striking alternately against the piston pin circlips:
Axial thrust in the piston pin is always the result of misalignment between the axis of the piston pin and the crankshaft axis. As described, bending or twisting of the connecting rod and asymmetry of the connecting rod are the most common causes. Excessive connecting rod bearing clearances (connecting rod bearing journal on the crankshaft) can cause a lateral pendulum movement of the connecting rod, particularly at lower engine speeds. The piston pin is skewed as a result in the connecting rod eye and is pushed back and forth in the piston pin bore due to the pendulum motion. The piston pin strikes against the piston pin circlips as a consequence.
- Piston top land has impact marks in the tilting direction (Fig. 1).
- The piston skirt displays a more pronounced running pattern to the top and bottom than in the middle of the skirt.
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.