Copyright Goodheart-Willcox Co., Inc. Auto Fundamentals 212 better flow of current from the center electrode. Air is more easily ionized (broken down from a nonconductor to a conductor) near the hotter center electrode. As a result, less volt- age is needed to create a spark, which lightens the load on the coil. In waste spark ignition systems, the waste spark travels from the side electrode to the center electrode. Since the cylinder is not under compression, an extra load is not placed on the coil. However, reverse firing can cause electrode damage to certain types of spark plugs. A technician must always make sure that the replacement plugs are designed for use in a distributorless system. Spark Plug Insulators Spark plug insulators must have special properties. They must resist heat, cold, chemical corrosion, and sud- den voltage changes. Insulators must also be resistant to vibration and physical shock. These insulator properties keep the high-voltage current arriving from the ignition coil from shorting to ground before creating a spark. A common material used for making spark plug insulators is aluminum oxide. The aluminum oxide is fired at high temperature to produce a glassy smooth, dense, and very hard insulator. The length, diameter, and location of the insulator have a direct bearing on the heat range of a particular plug. The top end of the insulator is often ribbed, or grooved, to prevent flashover, Figure 10-14. Spark Plug Shells The center electrode, surrounded by the insulator, is placed in a steel spark plug shell. The top of the steel shell is gen- erally crimped over to bear against a seal. The crimping process causes the shell to grip the insulator tightly and also forms a pressure seal at both the top and bottom of the insulator. This prevents combustion leaks and arcing. The side electrode is welded to the steel shell. The shell is threaded so it will screw into a threaded hole in the cylinder head. The shell’s threaded area will vary in length and thread pattern to conform to the cylinder head in which it is installed. The shell forms a seal with the head by means of a copper or aluminum gasket, or by a beveled edge that wedges against a similar bevel in the cylinder head, Figure 10-15. The thread seal is impor- tant, since this is an area through which a great deal of the plug’s heat is transferred to the head metal. The insu- lator is subjected to tremendous temperatures. In order to prevent burning, it must get rid of surplus heat. Spark Plug Heat Range Spark plugs are designed to operate at a certain tempera- ture, neither too hot nor too cold. If a plug runs too cold, it will collect deposits and become fouled. If it runs too hot (much above 1700°F, or 967°C), it will cause preigni- tion from hot carbon deposits igniting the air-fuel charge before the plug fires. The heat range is determined by the diameter and length of the insulator as measured from the sealing ring down to the tip, Figure 10-16A. It is important that the plug extend into the combustion chamber just the right amount. Gasket Cylinder head Tapered seat for sealing Goodheart-Willcox Publisher Figure 10-15. Spark plug thread seals. Not all spark plugs use a gasket for sealing purposes. Cold Hot Cold Type Short Heat Path Hot Type Long Heat Path Sealing ring Heat path Insulator Coolant A B Champion Spark Plug Company Figure 10-16. A—Cold-to-hot spark plug heat range. B—A cold type, short heat path and a hot type, longer heat path. Study the heat flow through both types.