Big Horn 18991 3/8" Type "C" Countersink, 5/32" Drill Hole Size. The Type “C” countersink is designed for use in all woods, plastics, and non- ferrous metals. The countersink is held in place with 2 hex socket set screws fastened into the flutes of the drill. This countersink can be mounted on taper point, brad point and regular point drills, The combination of a quality high-speed tapered drill with a countersink and stop collar, in conjunction with the correct plug cutter, is by far the best way to prepare fine work for wood screws. Tapered drills afford greater gripping power for screws than standard twist drills because they match the tapers of their corresponding screw sizes, allowing the screw to grip along its entire length. Our matching countersinks are made of high carbon steel and have four flutes for clean, accurate boring with minimum tear-out. They are adjustable along the length of the drill and can be tightened with two set screws. The stop collars are also adjustable for varying counterbore depths and can be set just behind the countersink cutters when a counterbore is not necessary.