Abstract: A hydraulic stop for use in drilling or countersinking operations comprises a rotatable body for rotating a tool about its axis. A cage adapted to contact the surface of a workpiece when the tool has penetrated into the workpiece by a predetermined distance actuates a trip mechanism comprising a plunger-operated hydraulic valve as the tool penetrates the workpiece more than the predetermined distance. The plunger is of adjustable length. When actuated the trip mechanism allows the body to shrink in length suddenly. The cage is axially adjustable in relation to the body to set the predetermined depth at which the trip mechanism actuates. A plurality of holes spaced around the circumference of the body to which the cage is mounted allow drainage of coolant whatever the axial position of the cage.