11 and the capped bottom end a series of two or more lug windows or key openings are spaced circumferentially around the key retaining housing 5 and each open radially therethrough at a position intermediately between the insert and a point where the top end of the compression spring would reside when fully compressed an annular key or lug recess is provided in the internal wall surface of the flow release sub at a position aligning with the windows along the common central longitudinal axis of the tools housing components so that each window faces radially outward into the recess a chamfer is defined at the upper end of the annular recess by a downward taper in the wall thickness of the hydraulic housing 4 at the bottom end thereof a lug or key 9 is disposed in each of the lug windows each lug or key 9 has a locked position in which an inner surface of the lug engages a cylindrical outer surface of a bottom portion of the piston below an exterior chamfered annular recess extending around the piston when the piston is in the running position shown in the drawings the piston 39 s chamfered annular recess has chamfers at the upper.