The ends of both contact are head treated steel and precision ground to a radius allowing proper clearance on the smallest hole the gauge will enter. Constant spring tension delivers uniform contact pressure. Double action handles are rigidly attached to the contact plungers and are automatically self-centering. Use the gauge to expand in the hole to be measured, lock it in place, and then take it down, and measure with micrometer or caliper to determine the final hole size. Assist in quick checking of holes, slots and recess up to 6 inch deep. Super quality, high rigidness, good wear resistance, in use. It is used to obtain the exact size of holes and slots, which is suitable for factories, schools, offices and other places.