These combination tools in TCT can be used in abrasive materials. They will drill countersink & finally counterbore a parallel hole. Designed to receive plug maker group 24. The drill diameter sizes are for a clearance hole only. For use with drilling machines or routers. T.C.T.1/4 in. Shank. Woodscrew Size. No. 8. Drill Diameter 3/16 in. Countersink Diameter 9.5 mm. Drill Length. 20 mm.