Asymmetrical piston wear pattern

Fig. 1:

• Asymmetrical wear pattern on piston over its entire height.
• Top land on left of piston above the piston pin bore and on opposite side at lower edge of piston rubbed bare.
• Uneven wear pattern on compression ring.

Fig. 2:
Piston running skew, with the wear mainly on the lower, right-hand edge of the piston at the recess for the cooling-oil nozzle and under the piston pin bore.

This type of asymmetrical wear pattern indicates that the piston has not been running perfectly straight in the cylinder bore and that the piston pin axis and the crankshaft axis are not parallel to each other. The piston rings cannot perform their sealing function properly due to the lack of proper contact with the cylinder. The hot combustion gases blow through and heat up the piston rings and the cylinder wall excessively. This weakens the oil film, which may result in a seizure due to lack of lubrication. As the piston is running skew in the cylinder, its reciprocating movement creates a pumping effect at the piston rings, which pumps oil into the combustion chamber and thus increases oil consumption. Under certain conditions an axial thrust can be applied to the piston pin, which can lead to wear or fracturing of the piston pin circlips (refer to the chapter entitled “Piston damage caused by broken piston pin circlips”).