C.R. LAURENCE 18CS CRL Brand 9/16" Countersink for 12 to 16 Screws. CRL Countersinks are 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" drill chuck.. Part Numbers Variations: 18CS