Whiteside 4 Flute, 82 degree Adjustable Countersink, High Speed Steel, 1/2″ Outside Cutting diameter, 13/64″ Inside Diameter (for 13/64″ center drill), for countersinking holes for a #10 size screw. These Whiteside countersinks are designed to slip over the drill bit and affix with dual set screws into the flutes. Adjust the drilling length by sliding the head up or down the bit. Use to countersink holes in wood, plastic, or aluminum for flat head screws. High Speed Steel series for longer life, especially in fiberglass or other abrasive materials. Sold together with the Taper Point Drill #10 which is ideal for better control on entry and exit. The Taper Point resists the rapid pull-in tendency of gelcoat and other hard, thin materials. They also break through less abruptly, which helps reduce blowout on exit.