Counterbore with a slow spiral drill fitted. Tungsten Carbide Tipped for use in abrasive materials. Dimensions: D=3/16 inch D=4.75 mm D2=3/8 inch D2=9.5 mm Shank Diameter=1/4 inch hex Shank Diameter: 1/4 inch Diameter Imperial: 3/16 Diameter Metric: 4.75 Diameter 2 Metric: 9.5