PRE-IGNITION DAMAGE -- HOLE IN HEAD OF PISTON
Holes in the piston crown form mostly in high compression engines with predominantly hemispherical combustion chambers. Specific combustion defects cause such rapid local overheating that material in the affected area becomes pasty. The inertia forces due to the axial piston movement and the fast flowing combustion gases cause removal of the soft material. As a result of the loss of strength in this area, the combustion pressure then causes mechanical failure of the remaining two-thirds of the piston crown thickness. This lower part of the hole opens out downwards. Such a rapid local overheating of the piston crown to a pasty condition can only be caused by pre-ignition. Combustion is initiated some time prior to the actual spark enabling the flame to operate on the piston crown much longer than usual. Tests have shown that the piston crown heats up locally to the melting point within a few seconds under continuous pre-ignition.
With pre-ignition, combustion is initiated by a glowing area in the combustion chamber when the mixture self-ignition temperature is exceeded. Probable sources are the sparking plug, the exhaust valve and deposits adhering to the combustion chamber walls.