the sizing of flow passageways 40 42 and of the control valve 38 and designing for a given closing force by allowing the diameter of the upper piston 10 to be increased independent of the diameter of the valve stem 14 in general the cross sectional area of the upper piston 10 is greater than the cross sectional area of the lower piston 12 while the cross sectional area of the lower piston 12 is greater than the cross sectional area of the valve stem 16 the differential between the cross sectional area of the upper piston 10 and the lower piston 12 determines the force generated by a given fluid pressure in the upper chamber 24 and in the lower chamber 26 the upper chamber 24 is defined by the top first side 11a of the upper piston 10 and the actuator housing 13 preferably the lower chamber 26 is defined by the bottom side 11b of the upper piston 10 and the outer peripheral surface 15 of the lower piston 12 and the actuator housing 13 the lower piston 12 seals against the upper piston 10 to prevent penetration of the high pressure fluid between the bottom side 11b of the upper piston 10 and the lower piston 15 and resultant flow losses now referring to fig2 of the drawings a partial cross sectional view of the present invention is shown an upper piston 10 is positioned directly above and contacts a lower piston 12 both of which are secured to an engine valve 14 specifically on the upper portion of the valve stem 16 the upper piston 10 and the lower piston 12 and the valve.