During engine operation, pistons can break as a result of a forced fracture or can suffer a fatigue fracture.
A forced fracture (Fig. 1) is always caused by a foreign body that collides with the piston while the engine is running. These foreign bodies could be parts of the connecting rod, crankshaft or valves, etc. that have been torn off. A forced fracture of the piston can also occur if water or fuel gets into the cylinder.
The fracture surfaces of a forced fracture appear grey. They are not worn down and display no line markings. The piston breaks suddenly without any fracture development.
In the case of a fatigue fracture (Fig. 2), line markings form on the fracture surface that reveal the starting point and the gradual progress of the fracture. The fracture surfaces are often worn to the point of being shiny. The cause for a fatigue fracture is overstressing of the piston material.
Overstressing can occur due to:
- knocking combustion,
- severe vibrations of the piston, for example if the piston head has mechanical contact with the cylinder head,
- material defects,
- excessive skirt clearance.
Excessive deformation of the piston pin due to overstressing (bending and oval deformation) causes cracks in the boss or cracks in the support. Furthermore, fatigue fractures can also stem from heat stress cracks on the piston crowns.
- 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).
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.
- 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.
- Impact marks on the piston crown (Fig. 1), cylinder head mating surface and both valves (not shown).
- Fracture in the direction of the piston pin due to vibrations and the effects of violent impact.
- Piston skirt has broken off in the lower oil ring groove, fracture surfaces display the characteristics of a fatigue fracture (Fig. 2).
The cause is an exceptionally fast sequence of hard impacts as the piston crown strikes the cylinder head, which subjects the piston to such violent shock vibrations that cracks are generated. The piston also no longer runs straight in the cylinder and subsequently strikes the cylinder wall with its skirt. On pistons with a lower oil control ring (Fig. 2) the skirt often breaks in the area of the lower oil ring groove.
- Excessive clearances in the connecting rod bearings or a worn-out connecting rod bearing, particularly in conjunction with over-revving when driving downhill.
- The gap dimension (the minimum distance between the piston crown and the cylinder head) was too small at TDC of the piston. The following scenarios may have caused this:
- Pistons with an incorrect compression height. During engine reconditioning, the mating surface of the cylinder block is often reworked. If pistons with the original compression height are then refitted after machining, the piston protrusion may be too large. This is why pistons with a reduced compression height are available for repairs, enabling the piston protrusion to be kept within the tolerance range specified by the engine manufacturer.*
- Insufficient thickness of the cylinder head gasket. Many manufacturers provide cylinder head gaskets with different thicknesses for the same engine. This is necessary to compensate for component tolerances during production, and it also allows adaptation of the piston protrusion during repairs. For this reason, when carrying out repairs ensure that only cylinder head gaskets with the prescribed material thickness are used. This ensures that the specified gap dimension will be achieved after the repair. If the cylinder block is reworked or replaced during repair work, the thickness of the gasket must be re-determined depending on the piston protrusion in accordance with the engine manufacturer’s specifications.
* Motorservice supply pistons with a reduced compression height (KH-) for many diesel engines. For details please refer to the “Pistons and Components” catalogue.
Checking the freedom of movement by turning the engine by hand when it is cold does not guarantee that the piston will not strike the cylinder head when the operating temperature is reached. Reason: the piston and connecting rod increase in length as a result of thermal expansion, which reduces the gap between the piston crown and the cylinder head. Particularly on engines in commercial vehicles with large piston compression heights, the differences can be significant and reduce the freedom of movement of the piston at TDC by several tenths of a millimetre.