Spark Plug Gapping Tool
A tool used to measure and adjust the gap between a spark plug's center and ground electrodes.
A spark plug gapping tool (often called a gap tool) is a measuring and adjustment tool used by technicians to set the distance between the spark plug's center electrode and ground electrode. This distance (the gap) is specified by the vehicle manufacturer and is critical for proper engine combustion. Gapping tools come in coin-style, wire-loop, or feeler gauge designs.
An incorrect spark plug gap will cause engine performance issues. If the gap is too small, the spark will be too weak to ignite the fuel mixture efficiently, causing rough idling and misfires. If the gap is too large, the ignition coil may not produce enough voltage to jump the gap under high combustion pressures, leading to misfires under load. Modern pre-gapped plugs should still be checked, as they can be bumped and closed during shipping.
When adjusting the gap on modern platinum or iridium spark plugs, extreme care must be taken. The center electrodes of these plugs are coated with precious metals and are extremely thin and brittle. Prying against the center electrode with a coin-style tool can easily snap the electrode tip, ruining the plug. Specialized gapping pliers that only bend the ground strap should be used instead.