A counterbore hole is defined as a cylindrical and flat-bottomed hole that usually enlarges into another coaxial hole. A counterbore hole is typically used when a fastener, such as a socket head cap screw or fillister head screw, is required to sit flush with or below the level of a workpiece's surface.
BS 4185 is the series that discusses standards for machine tool components. BS 4185-13 is a part of the machine tool components series that specifies dimensions of counterbored holes for use with hexagon socket head cap screws complying with the requirements of BS 416-1 of sizes ranging from M2.5 to M30.
BS 4185-13 on counterbored holes dimensions is useful for:
A counterbored hole is essentially a pilot hole consisting of a long and narrow threaded channel at the bottom and a shallower but wider channel at the top.
BS 4185-13 provides you with all the requirements and dimension specifications for counterbored holes with hexagon socket head cap screws. Following dimensions specifications, you will be able to design the counterbored holes for use with hexagon socket head cap screws.