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Typical Damage Appearance
Scoring is typically on one side of the piston (see piston on right in photo), and good skirt on other side (see piston on left in photo). An originally good skirt bearing panel has narrow, clearly limited longitudinal friction marks superimposed. The ring zone is usually undamaged.

Assessment

Unburned fuel which has condensed on the cylinder walls has locally weakened the oil film such that in narrowly limited strips a dry run occurred between cylinder and piston, which always results in long narrow friction marks.

Possible Causes
1) Combustion disturbances due to faults in fuel injection system or carburetor or due to faulty ignition (misfiring);

2) Loss of compression through the valves or cylinder head gasket;

3) Choke too long in operation.

Courtesy of United Engine & Machine Company

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