Abstract: A centering apparatus for a free piston Stirling engine including a pressure regulator valve which connects a passageway between the work space and the back space when both (a) the average pressure in the back space exceeds the average pressure in the work space as a result of the piston deflecting a spring, and simultaneously (b) the instantaneous pressure in the back space exceeds the instantaneous pressure in the work space. A diaphragm spans across an actuator housing cavity, dividing the cavity into an upper chamber and a lower chamber. A connector valve is linked to the diaphragm for opening and closing the valve upon displacement of the diaphragm. When the average back space pressure exceeds the average work space pressure, the diaphragm is displaced to open the valve, connecting the work space and the back space in fluid communication. A check valve permits the flow of gas in only one direction: from the back space to the work space.