Some definite knowledge of spark plug construction quality, and care, will be found very useful to the average motorist in purchasing new plugs, and keeping those in present use, in good condition. A good plug properly constructed should outlast the life of the motor. When purchasing new plugs, first examine the old plug and get one of the same length. This is very important as spark plugs are made in as many different lengths as required by high and low compression motors. High compression motors have a small low walled combustion chamber, while low compression motors usually have a spacious high wall chamber and require a longer plug, whereas if the long plug is used in the high compression motor it may be put out of commission by the ascending piston. Next determine the size of the plug and the gauge of the thread. The majority of motors use the ³⁄₄ inch plug, with the S. A. E. thread, while a few still use the A. L. A. M. thread which is much finer gauged. Another point to be remembered is that it is an unwise expenditure to purchase cheap plugs because the intense heat and pressure that they are subjected to and required to stand, demands that they be made of the highest quality of material and workmanship.

Fig. 104 shows the sectional construction of a spark plug costing from one dollar to one dollar and fifty cents. No. 1, the terminal, is designed to fit all connections. No. 2 nut which holds electroids firmly in place. No. 3 represents round edged shoulders which prevent the plug from short circuiting on the outside. No. 4 is a heavy electroid which will not break or burn. No. 5 is an extra heavy insulator which insures a good spark in case the outer porcelain insulator becomes broken or cracked. No. 6 is a bushing which holds the insulator firmly in place from the top. No. 7 is a high compression washer which allows for upward expansion and makes an even seat for the bushing which holds the insulator in position. No. 8 is a massive porcelain insulator designed to withstand a high temperature without cracking. No. 9 is a copper asbestos washer that allows for the downward expansion of the insulator. No. 10 is the shell casting which holds and protects the insulator. No. 11 are rounded corners which will allow the plug to be screwed down flush without coming into contact with the curved walls of the cup containers. No. 12 is a high compression washer which prevents all leakage. No. 13 shows elastic cement which strengthens the lower construction of the insulator and prevents the compression from escaping through the center of the insulator. No. 14 is a hardened polished steel tipped electroid. No. 15 is a bent polished steel electroid dipped on each side of the spark in order to prevent oil from running down from the shell casting and closing the spark gap. No. 16 represents an extended center electroid which prevents any oil that may have lodged on it from stopping at the spark gap.

Spark Plug Cleaning.—To insure a smooth running motor and a good spark, the spark plugs should be cleaned at thirty day intervals. It is not always necessary to disassemble them at this time as the carbon usually collects and bakes on the metal casting shell and can be removed by running a thin knife blade or finger nail file around the inner surface. However, when the insulator becomes pitted or carbon burnt the plug should be disassembled and the insulator wiped clean with a cloth dampened in kerosene. Never immerse the insulator in kerosene, as this will loosen the cement around the center electroid and cause the plug to leak compression. The shell may be immersed. It is then wiped dry and the inside surfaces scraped or rubbed with a piece of sand or emery paper to dislodge the carbon pits. After all parts have been thoroughly dried the plug is reassembled, using new washers.