The Proform Telescoping Gauge Set allows you to quickly measure the inside diameter of everything from basic bore diameter, to grooves, to hard-to-reach recesses. Simply insert the gauge into the area, turn the knob and the spring-fed arms will snap to the outer edges. Now, tighten the knob to lock the inside measurement then use a Proform Micrometer
to obtain the desired measurement. The set includes six gauges with sizes ranging from 5/16'' to 6'' and a durable carrying case.