Abstract: A method and apparatus for controlling the reciprocal movement of a free, gas-driven displaced in a cooling engine are disclosed. Mechanical stops in the form of a crank mechanism prevent axial overshooting of the gas-driven displacer in each direction. A uni-directional magnetic "detent" is established for the displacer at predetermined location beyond the top dead center and bottom dead center portions of the cooling cycle. Each uni-directional magnetic "detent" provides a magnetic retention force to hold the displacer from moving until a pre-determined pressure differential is established across the displacer's drive piston. The magnetic "detents" are formed by magnetic elements that operate in cooperation with the crank mechanism.