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A counterbore is a cylindrical flat-bottomed hole that enlarges another coaxial hole, or the tool used to create that feature. A counterbore hole is typically used when a fastener, such as a socket head cap screw, is required to sit flush with or below the level of a workpiece’s surface.
Counterbores are made with standard dimensions for a certain size of screw or are produced in sizes that are not related to any particular screw size. In either case, the tip of the counterbore has a reduced diameter section referred to as the pilot, a feature essential to assuring concentricity between the counterbore and the hole being counterbored.