154 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. on production vehicles for many years, although they are available as aftermarket items. Most secondary wires are 6 to 8 millimeters thick. The actual conductor is only a small part of the wire. The majority of the wire’s diameter is made of heavy insulation, such as rubber, neoprene, plastic, or other non-conducting materials. Secondary wires have a metal connector on each end. The internal end of the connector is attached to the con- ductor. The exposed end of the wire is designed to tightly connect to the distributor tower, coil tower, or spark plug, as applicable. Each wire end has a rubber or neoprene boot to keep out water and other contaminants. Some wire ends are specially made to fit certain engine designs, such as the ones in Figure 8-22. Caution: Spark plug wires with copper or aluminum cores are designed for off-road use only. They should never be used to replace resistance wires as they create interference that can affect engine and vehicle operation. Spark Plugs The spark plug is an ignition device that provides a gap in the combustion chamber across which the high-voltage current in the secondary circuit can jump. This jump is a spark that ignites the compressed air-fuel mixture. The spark plug also provides an insulated path for the secondary current into the combustion chamber. The spark plug must be able to withstand extremes of combustion-chamber heat and chemical attack during this process. The typical spark plug may be expected to last from 15,000–30,000 miles (24,000–48,000 km) up to 100,000 miles (160,000 km). If not periodically changed, however, the spark plugs can cause performance problems. Conventional Spark Plugs A cutaway view of a typical spark plug is shown in Figure 8-23. A typical plug is composed of the center electrode, ground electrode, and insulator. The center electrode conducts the spark into the combustion chamber. Typical materials used to make center electrodes include copper and nickel alloy. Platinum, although expensive, is sometimes used. Most center electrode assemblies contain a spring under compression to ensure physical contact between the electrode’s metal parts at any temperature. Spark plugs have a small resistor built into the center electrode. The resistor reduces static interference caused by the magnetic field created when the current jumps the gap. Figure 8-23. This cutaway shows the internal components of a conventional, resistor-type spark plug. Most spark plugs are constructed in a similar manner. (Bosch) Terminal nut Terminal stud Insulator Insulator tip Center electrode Ground electrode Current-leakage barrier Swaged and heat-shrunk fitting Special conductive glass seal Captive outer gasket (in the case of flat seal) Connection thread Figure 8-22. Some secondary wires have special boots. These boots are used when the plug is in a hard-to-reach spot. Since the spark plugs in many new engines are placed in the center of the combustion chamber, boots such as these are becoming more commonplace.