Starrett S579HZ Self Centering Telescoping Gauge With 2 Telescoping Arms Set, 5/16"- 6" Range, 2-3/8 - 3-1/4" Handle Length (6 Pieces)
579 Series Telescoping Gages are for determining the true size of holes, slots and recesses up to 6".  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slightly "rocked" in the hole being measured to ensure that the tool is on the proper diameter before it is locked and withdrawn.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.