The cracking direction of piston top is generally vertical to the axial direction of piston pin hole.

1. The overload run of engine cause the over deformation of piston and relative components and result in the cracking of piston top.

2. The abnormal operation of combustion system or overload of engine cause increased thermal gradient of piston top and result in thermal cracking of piston top.

3. The inferior of piston materials cause the decreasing of high temper-ature  strength.

4. If the combustion contains sulfur, vanadium and other elements, it may give rise to high-temperature corrosion and accelerate the seriousness of cracking.