A hydraulic stop (2) for use in drilling or countersinking operations comprising a rotatable body for rotating a tool (6) about its axis and a cage (20) adapted to contact the surface of a workpiece (8) when the tool has penetrated into the workpiece by a predetermined distance and to actuate a trip mechanism comprising a plungeroperated hydraulic valve (30, 32) as the tool (6) penetrates the workpiece more than the predetermined distance, wherein the plunger (28) is of adjustable length. When actuated the trip mechanism allows the body to shrink in length suddenly. The cage (20) is axially adjustable in relation to the body to set the predetermined depth at which the trip mechanism actuates. Drainage means is provided comprising a plurality of holes spaced around the circumference of the body to which the cage (20) is mounted to allow drainage of coolant whatever the axial position of the cage (20).