Starrett 579E-12 Self Centering Telescoping Gauge, Two Telescoping Arms, 2-1/8"-3-1/2" Range,

Telescoping gauge is used for determining the true size of holes, slots, and recesses up to 6". The ends of both contact is hardened and ground to a radius to allow proper clearance on the smallest hole the gauge will enter. This tool must be slightly "rocked" in the hole being measured to ensure that the tool is on the proper diameter before it is locked and withdrawn. The final hole size is obtained by measuring over the gauge contact with a micrometer.