Spark plug gauge

A spark plug gauge is used to determine the distance between the center and ground electrodes of a spark plug . This distance must be accurate to within 0.1 mm. It increases with increasing operating time and can be corrected by slightly bending the earth electrode. This is achieved by gently knocking on a hard surface, whereby the distance must never be too small.

Most spark plug gauges have a conical hole that you can use to bend the ground electrode open again if, despite all caution, it has been bent too much inward. The gauge is available with both inch and millimeter scales; the value may have to be converted to set the correct distance.