A portion of a drill assembly operated by a supply of compressed fluid comprises a backhead, a cylinder portion, a hollow elongate wear sleeve and a piston. The backhead has a proximal end connectable to the supply, an axial bore and an open distal end having an integrated cylinder portion defined therein. The backhead has passages extending between the axial bore and the outer surface of the backhead. The cylinder portion is aligned with and in selective fluid communication with the backhead. The hollow elongate wear sleeve surrounds the cylinder portion and is connected to the backhead. The piston is housed by the wear sleeve and has a proximal end shaped to fit within the cylinder portion. The piston is slideably movable along the wear sleeve and the cylinder portion in response to compressed fluid conveyed through the backhead. When the drill assembly is in a drop open position, the proximal end of the piston is radially spaced apart from a nearest surrounding surface.