SMALL HOLE GAUGE-4PCS/SET WITH FITTED CASE KNURLED KNOB AT END OF HANDLE ADJUSTS GAGE TO HOLE. SIZE READINGS OBTAINED BY MEASURING OVER CONTACT POINTS WITH A MICROMETER. GAGING SURFACE IS HARDENED GAGING SURFACE IS FULL ROUND WITH A FLAT BOTTOM. YOU CAN USE IT IN SMALL SHALLOW HOLES, SLOTS AND GROOVES.