In the case of seizures due to overheating, the oil film breaks down as a result of excessively high temperatures. Initially this causes mixed friction with individual friction marks. The material at the friction marks then heats up further and the piston loses all lubrication. The seizure areas have a dark discolouration and are badly broken up. Depending on the cause of the damage, the seizure due to overheating starts at either the piston skirt or the piston head.
- Severe seizure starting from the piston head and lessening as it progresses towards the skirt end.
- Seizure marks are distributed around the entire circumference of the piston head.
- Surface of the seizure areas has a dark discolouration with severe scoring marks, and has broken up in places.
- Piston ring running surfaces show signs of seizure, which decrease in intensity towards the oil control ring.
The piston head has been heated up so much as a result of extreme thermal overload that it has bridged the running clearance and destroyed the oil film. This caused a combination of seizure marks due to insufficient clearances and due to a lack of lubrication all around the piston head. A general lack of clearance due to insufficient piston installation clearance can be excluded as the possible cause, because in this case the damage would start in the skirt area (refer to the chapter entitled “Seizure due to insufficient clearance on the piston skirt”).
- Extended high loads during the running-in phase of the engine.
- Overheating due to faults in the combustion process.
- Malfunctions in the engine cooling system.
- Malfunctions in the oil supply (pistons with oil cooling or with oil cooling gallery).
- Bent or faulty oil injection nozzles, which insufficiently cool the piston with oil from underneath.
- Use of wrong sealing rings on the liner flange of wet cylinder liners (refer to the chapter entitled “Cavitation on cylinder liners”).
- The piston skirt has seized on both sides.
- The surface of the seizure marks has a dark discolouration and is rough and severely broken up.
- The piston ring zone is often only slightly damaged or not damaged at all.
Severe overheating of the engine has caused the lubrication on the cylinder sliding surface to completely break down. This has caused a seizure due to lack of lubrication with a heavily broken up piston skirt. The damage is centred on the skirt area; there are no seizure marks around the piston head. It is thus possible to exclude engine-based overstressing caused by abnormal combustion.
- Overheating engine caused by the following malfunctions in the cooling system:
- lack of coolant
- defective water pump
- faulty thermostat
- torn or slipping V-belt
- inadequately ventilated cooling systems.
- On air-cooled engines: overheating due to dirt deposits on the exterior of the cylinders, broken cooling ribs or failed or compromised cooling air ventilation.