U.M.R. Engines

PISTON - CRACKED
Cracks in the top of a piston (crown) in petrol engines are usually the result of excessive combustion pressure caused by excessive compression or over advanced ignition timing. These conditions cause excessive combustion pressure, which in turn causes the piston crown to crack as the piston is operating outside the pressure it was designed to work under. In diesel engines this damage can be caused by a condition called thermal fatigue.

Thermal fatigue occurs when an engine is consistently operated under full heavy load followed by light load. The constant drastic changes in combustion temperature eventually results in thermal cracks of the piston crown.

Piston skirt cracking is usually a result of constant excessive loading of the engine and high mileage fatigue or in some cases faulty piston design. The manufacturer usually corrects the later by supplying a superseded part.

Incorrect fitting of pistons to rods can cause stress fractures, which develop into serious skirt cracking early in the life of a repaired engine. Piston skirt cracking is not a common engine fault and IS ALMOST TOTALLY ELIMINATED OUT OF MODERN PISTON DESIGN

Cracked ringlands are usually caused in detonation or pre-ignition. Detonation causes excessive combustion temperature and pressure. By design the ring lands are one of the weaker positions on a piston and can crack under this type of stress. The land can completely crack away in sections. Rings will also crack under these conditions.

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