Starrett telescoping gages are used for determining the true size of holes, slots, and recesses up to 2-1/8" (54 mm). The ends of both contacts are hardened and ground to a radius to allow proper clearance on the smallest hole the gage will enter. These tools 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slightly "rocked" in the hole being measured to ensure that the tool is on the proper diameter before it is locked and withdrawn.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 gage contacts with a micrometer.
The No. 579 Series telescoping gages have handles that are rigidly attached to the contact plungers and are automatically self-centering. Also, the No. 579 gages have two telescoping contacts.
The handles are approximately 2-3/8" (60 mm) long.
Constant spring tension gives uniform contact pressure and both plungers are easily locked at any desired setting.