The really outstanding parts of the spark plug, in the true sense, are the connecting center-pin and the insulator made from Al2O3 - ceramics with a lead-free glazing. The ribbing should prevent the connection from being earthed. Towards the bottom there was previously a larger (e.g., 21 mm) hexagon, or a double hexagon, nowadays, because of the cramped space, it is smaller (e.g., 16 mm).
Going on further, there is the flat- or conical sealing seat, the former with an unloseable sealing ring. Right at the bottom there are up four welded-on earthing electrodes. Then the center electrode which is connected to the inside of the insulator appears again. During the manufacturing process this part is pressed into the metal threaded seating, this is then beaded and shrunk.
What it can look like below the threading can be seen in the above picture. The plug on the left is called a 'hot' plug, because the heat from the center electrode cannot be passed directly to the threading, thus to the coolant in the cylinder head. The better this is, e.g., seen in the right hand plug, the quicker the heat transfer is to the cylinder head and the 'cooler' it remains. The heat-index-value is then said to be low.
Earlier, the more sporting the engine was and the harder the car was driven, the greater was the role of the heat value. The engine tuners carried out tests with plug-sets having various heat-indexes. Should the plugs transfer too much heat off, they fall below the self-cleansing temperature. The plugs become carboned up and, at least those in the affected cylinders, stopped working properly. One radical example is the sports car crawling along behind a streetcar.
Should the plug become too hot, possible self-ignition is triggered, this strains the entire crank mechanism. There is also the possibility of a nice, round hole in the piston or a spot of melting. Therefore the preferred range of testing to find the correct heat-value, is that where the demands on the engine are highest. Apart from the full throttle orgy, this can also be the high strain range at low RPM. 05/12