Usage:
All Caterpillar Gas Engines
Three 18 mm spark plugs with the lower half of their metal shells removed. Illustration of the effects of Corona.
The photograph shows three 18 mm spark plugs with the lower half of their metal shells removed. Examine the stains (A) on the ceramic part of the plug just above the metal shell. Some might conclude the traces on the ceramic are the result of combustion gasses leaking between the shell and the ceramic insulator. This is not the case. The traces are the result of a phenomenon called Corona. Corona is caused by an electric field that is the result of the high energy ignition system operating as it is designed. Please note that the lower half of the ceramic (B), which is normally inside the metal shell, is still pearly white and clean. This would not be the case if the plug were actually leaking.