During the course of normal operation, the electrodes of a spark plug will erode away and must be re-gapped as needed. But there comes a point at which the electrodes will have worn down too much, making the plug unserviceable. This simple go-no-go gauge makes it easy to determine if you plugs have reached the end of their useful life. Simply gap your plug per manufacturer's recommentation and then check to see if the electrodes fit through the hole of the gauge. If they enter the hole, then your plug is too worn and must be replaced. It's that simple.