 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.Because the spark plug also seals the combustion chamber of the engine when installed, seals are required to ensure there is no leakage from the combustion chamber.