Caps are mainly required for motorcycles, scooters, small engines and marine applications. They are used to connect each spark plug to a high tension (HT) ignition lead; typically by screwing them onto the end of the cable. Occasionally, caps can be applied to 4-wheel vehicles as well, in which case they are usually pre-assembled. The caps feature a resistance element made of a high-quality ceramic material. This ensures a constant resistance, with small deviations in case of overload. The housing is made of phenolic resin with extremely high heat resistance and tensile strength. Special rubber bushings at both ends prevent electrical losses due to moisture.