A reverse chamfering and countersink stop tool allows the required diameter of a countersink in a workpiece to be maintained in areas that have limited accessibility for the user assembling a fastener to the workpiece. The tool controls the dimension required for proper installation of fasteners under such conditions. The tool can also be used for other applications that require control of the diameter of a chamfer, and allows the diameter of the chamfer/countersink to be set outside the point of use for maintaining proper tolerances. The tool utilizes a modified reverse countersink cutter that is captured inside an externally threaded adjustable stop housing. The captured cutter is movable both axially and rotationally, and is driven by a drill equipped with a pilot. The stop position of the cutter is controlled by a series of nuts that are located and threaded on the exterior of the housing.