Editing Reading spark plugs (section)
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====Insulator tip==== On modern (post 1930's) spark plugs, the tip of the insulator protruding into the combustion chamber is the same sintered aluminum oxide (alumina) ceramic as the upper portion, merely unglazed. It is designed to withstand 1,200° F and 60,000 volts. The dimensions of the insulator and the metal conductor core determine the heat range of the plug. Short insulators are usually "cooler" plugs, while "hotter" plugs are made with a lengthened path to the metal body, though this also depends on the thermally conductive metal core. Older spark plugs, particularly in aircraft, used an insulator made of stacked layers of mica, compressed by tension in the center electrode. With the development of leaded petrol in the 1930s, lead deposits on the mica became a problem and reduced the interval between needing to clean the spark plug. Sintered alumina was developed by Siemens in Germany to counteract this. Sintered alumina is a superior material to mica or porcelain because it is a relatively good thermal conductor for a ceramic, it maintains good mechanical strength and thermal shock resistance at higher temperatures, and this ability to run hot allows it to be run at "self cleaning" temperatures without rapid degradation. It also allows a simple single piece construction at low cost but high mechanical reliability.
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