C.R. LAURENCE CS4 CRL Brand 4 Piece Countersink Set for No. 6 to 16 Screws. CRL Countersinks were specifically designed to cut round countersunk holes in aluminum. They cut faster than standard countersinks and leave a finished hole every time. Smooth drilling action from single cutting edge prevents chatter and fluted edges. The 1/4" (6.3 mm) shank diameter fits any 1/4" or 3/8" (6.3 or 9.5 mm) drill chuck.. Part Numbers Variations: CS4