A spark plug that does not fire, resulting from a narrow gap, will appear black or wet when inspected. The symptoms of narrow gap include a noticeable (continuous) cylinder miss, hard starting if all the plugs have narrow gaps, rough idle and engine hesitation. The duration of the spark has less travel distance, thus not remaining hot enough with sufficient charge to ignite the air-fuel mixture. When the spark plug gap is too narrow, or under specifications, the amount of room needed for the air-fuel mixture between the hot tip and the ground strap is decreased. Reducing the spark plug gap is accomplished by tapping down the outer ground electrode with a small hammering tool, or prying it open with a thin blade, so that an accurate gap can be set with a spark plug tool. Feeler gauge blades, sliding bevel, wire type and pliers tools exist to gauge and set the distance between the electrode contact points. Several tools are available to gap spark plugs. Spark plugs generally produce 10,000 to 30,000 volts of arc between the electrode points. For example, a typical plug gap is 0.035, or thirty-five thousandths of an inch. The gap measurement will be indicated in thousandths of a inch. The gap, the distance between the 'Hot' and 'Ground' electrode points, is set and listed by the manufacturer for optimum performance. Heat range indicates the length of time needed for heat to be drawn away from the plug tip. Spark plug specifications, which include type, heat range and gap, will be found in the owner's repair manual. A high voltage spark jumps the gap, producing ignition fire. A copper core ground electrode that serves as a ground strap, either straight or tapered, arches over the tip to provide a gap. The center electrode ends in a gold or palladium nipple-like tip. The spark plug also has a steel jacket, which allows the threaded end of the plug to be torqued into the cylinder head. The tip continues down as the center electrode through an insulated core composed of a ceramic housing, usually made of porcelain. The basic construction of a spark plug uses an upper contact tip that receives voltage from the ignition system through a spark plug wire. A spark plug that has an improper gap will show certain signs and symptoms. Setting the gap on the spark plug electrode determines the voltage length and duration of the spark. A spark plug has a relatively simple construction and is nearly maintenance-free, except for the adjustment (gap) of its electrode and periodic cleaning or replacement. Automobile vehicles use spark plugs to ignite the air-fuel mixture inside the cylinder for the purpose of combustion.

Improperly gapped spark plugs