MS Motorservice provides readers with an in-depth guide to piston seizures and the role lubrications plays.
What happens if the piston, piston rings and running surface of the cylinder liner rub against each other without any lubrication? What causes seizure on parts of the piston skirt? Why is it absolutely essential for the air baffles of aircooled engines to be installed professionally? What happens if the engine is run with an over-rich mixture and what are the consequences of combustion faults? Let’s answer these questions.
Seizures due to lack of lubrication can occur generally, i.e. even if there is sufficient clearance between the cylinder and the piston. In the process the oil film breaks down (often only locally) because of the high temperatures or because of fuel flooding. In these areas, the surfaces of the piston, piston rings and cylinder running surface then rub against each other without any lubrication, which soon causes seizure with a severely worn surface.
A similar situation arises if there is an insufficient lubricating film between the piston and the cylinder due to a lack of oil.
Clues that a lack of lubrication has caused a seizure
The oil film is destroyed altogether (Fig.1):
Seamless areas of narrow seizure marks, mainly on the piston skirt, showing a severely worn and darkly discoloured surface.
There is a lack of oil (Fig.2):
Identical to those described above, apart from the discolouration of the surface. The surface of the seizure areas has an almost pure metallic finish, with no dark discolouration. As the lack of oil affects the entire surface of the cylinder, there are often seizure marks on both the pressure side and the anti-thrust side in the piston, in many cases even in the early stages.
Seizure due to lack of lubrication on the piston skirt (Fig.3)
- Seizure marks on the piston skirt on the pressure side, which in some cases extend into the piston ring zone.
- Slight seizure marks on the anti-thrust side.
- Surface of the seizure areas is light-coloured and has an almost pure metallic finish.
Damage Assessment
There was an acute lack of lubricant between the piston and the cylinder bore. The fact that the surface of the seizure areas has an almost pure metallic finish indicates that the oil film was present but significantly weakened at the time of the seizure. Due to the limited extent of the damage, this concerns a temporary lack of oil or the early stages of damage. The damage would have been even more severe if engine had been operated further (Fig.4).
Note: With this type of seizure due to lack of lubrication, the damage area is always located in the area of the piston skirt where the normal wear pattern would have formed on an undamaged piston after running in.
Possible causes
Insufficient lubrication caused by:
- Insufficient engine oil.
- Oil pressure in the engine too low (oil pump, pressure relief valve, etc.): not enough oil emerges at the crankshaft bearings. This means that not enough lubricating oil is supplied to the cylinder running surface, which is lubricated with splash oil and centrifugal oil from the crankshaft.
- Failure of the oil injection jet for cooling the piston.
