since 1940

Cracked aircraft spark plug insulator (electrode insulator)

1. Thermal shock from preignition or water ingestion (float planes).
2. Mishandling and cleaning.
3. Detonation.

Mechanism by which Thermal Shock Cracks the insulator

Published in the 1955 Aircraft Spark Plug and Ignition Conference Report page 69 by Mr. Eaton (Champion)

The thermal shock that takes place on the insulator occurs during rapid heating of the outside of the insulator nose surface.

The inner bore of the insulator is colder than the external surface because it is in close proximity to the center electrode which operates at a lower temperature. The actual operating temperature of the bore of the insulator is controlled by the thermal conductivity of the space between the bore of the insulator and the center electrode. If the space between the center electrode and the bore of the insulator becomes filled with deposits, then you increase that thermal conductivity and you can pass more heat between the bore of the insulator and the center wire for a given temperature drop. The result is that, during the rapid heating cycle, the center electrode does not want to change in temperature very much. It stays behind in temperature rise.

The center wire exerts influence on retaining the bore temperature of the insulator more nearly like the center wire temperature during the rapid heating cycle. However, during the rapid heating cycle, the outside surface of the insulator nose has to go up because that is the surface where the heat is being transmitted to the insulator. The result is you establish a greater temperature differential between the bore of the insulator and the external nose surface. That increased temperature differential across the insulator nose sets up tensile stresses in the bore of the insulator. If these tensile stresses are greater than the tensile strength of the insulator at that temperature, the crack will start in the bore of the insulator. With continued cycles of thermal shock of that nature, or the more severe pre-ignition shocking, that crack will open up more and more and progress from the inner bore to the outer surface and you will see it as a crack on the nose.

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