Piston fracture in the piston pin boss
DESCRIPTION OF THE DAMAGE
- Formation of a cleavage fracture that leads to the piston crown. This results in the piston being split into two parts (Fig. 1).
- Boss fatigue crack in the centre axis of the piston pin bore (Fig. 2 and 3).
DAMAGE ASSESSMENT
Boss fatigue fractures arise as a consequence of mechanical overstressing. The constant overstressing of the piston material increasingly results in alternating bending stresses and material fatigue. This process can be accelerated if there is no sufficient oil supply: an incipient crack in the piston pin boss will then spread even under normal loads, and will cause the piston to split.
POSSIBLE CAUSES FOR THE DAMAGE
- Abnormal combustion, in particular spontaneous combustion caused by ignition delay.
- Excessive or inappropriate use of starting aids during cold starts.
- The cylinder has filled up with water, fuel or oil whilst the engine is stopped (hydraulic lock).
- Performance enhancements (e.g. chip tuning) with use of the standard production piston.
- Incorrect or weight-reduced piston pins. The piston pin is deformed to an oval shape, placing excessive loads on the piston pin bed in the process.
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