Warning: Missing argument 2 for wpdb::prepare(), called in /data/16/2/156/48/2971537/user/3295596/htdocs/wp-content/plugins/reference_library/single-wordphrase.php on line 128 and defined in /data/16/2/156/48/2971537/user/3295596/htdocs/wp-includes/wp-db.php on line 1202

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.