Reasons for cracking:

1. Thermal fatigue: The piston crown is subjected to high, cyclic thermal stresses from the combustion process (rapid heating and cooling each cycle). Repeated thermal cycling causes thermal fatigue cracking, especially at the combustion bowl rim, valve recesses and the crown edge.
2. Thermal stress concentration: Sharp corners, changes in section, and the combustion bowl shape create stress concentrations where cracks initiate.
3. Overheating: Insufficient piston cooling (blocked/ reduced cooling oil flow) raises the crown temperature, increasing thermal stress and promoting cracking.
4. Mechanical/ combustion loading: High combustion pressures and mechanical loading, combined with thermal stress, cause fatigue.
5. Material/ manufacturing defects: Inclusions, porosity, or poor casting/ forging quality in the crown material.
6. Corrosion/ erosion: Combustion products (sulphur, acids) and erosion at the crown surface weaken the material.
7. Excessive/ abnormal combustion: Detonation, pre-ignition, or overloading increases the thermal and mechanical loading.

Consequence/ extent of consequence:

• A fine hairline crack in the crown is often of limited consequence initially (it may not affect operation immediately), but it can propagate under continued thermal/ mechanical cycling.
• A localised, shallow crack may be repairable (by grinding/ welding) if it is within limits and not in a highly stressed area.
• A deep or extensive crack, or a crack in a critical area (combustion bowl rim, valve seat), is serious: it can propagate, cause the crown to fail, allow cooling oil to leak into the combustion space, or cause a catastrophic piston failure. Such cracks are of major consequence and require the piston to be renewed or the crown repaired/ replaced.

• Fine hairline crack: If the crack is very fine and within the maker's limits (not in a critical area), it may be monitored (checked at each overhaul) and the piston kept in service, or the crack may be ground out (a small groove) to remove the stress raiser and prevent propagation. If it is in a critical area or propagating, the crown is renewed.
• Localised and shallow crack: A localised, shallow crack (within the maker's repair limits) can be repaired by grinding out the crack to a smooth groove and welding (using the correct welding procedure/ filler for the crown material), then machining/ grinding flush and checking with dye-penetrant. The repair must be approved by the maker/ Class. If the crack is too deep, extensive, or in a critical area, the piston crown is renewed.
• In all cases, the cause (overheating, cooling failure, abnormal combustion) must be investigated and rectified to prevent recurrence.