A spark plug gapper is a tool used to measure and adjust the gap between a spark plug's electrodes. This gap is the distance the spark must jump to ignite the fuel-air mixture in the cylinder. If the gap is too small, the spark will be weak, leading to rough running and poor fuel economy. If the gap is too large, the ignition system may not be able to generate enough voltage to jump the gap, causing engine misfires under load.
The most common gapper is the coin-style ramp gauge. It features a sloping edge that increases in thickness around the circumference. You slide the spark plug over the edge until it stops, reading the thickness mark on the face. Wire loop gauges are another style, using precise metal wire loops of standard thicknesses. To adjust the gap, the tool features a small hole or hook used to gently bend the ground electrode closer or further from the center electrode.