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A spark plugs insulator serves multiple purposes:
- It provides stability to the plugs center electrode.
- The upper portion of the insulator extends the plug above the cylinder head, making it more easily accessible and extends the electrical path to help prevent flashover. This upper portion may be ribbed (to further prevent flashover) or smooth.
- The bottom portion of the insulator, which supports the center electrode, protrudes into the combustion chamber. Its dimensions take part in controlling the heat range of the plug, or its ability to remove heat picked up during the combustion process.
Though various materials have been used for spark plug insulators through the years, the current contender is aluminum oxide. Most spark plugs now use a 1-piece ceramic insulator made from purified alumina powder. This modern day insulator provides many benefits over previous designs, including superior insulation or thermal conductivity. It also helps prevent overheating and provides exceptional strength to mechanical and thermal shock.