A double acting, miller cycle, reciprocating piston with dual rotary displacer, stirling engine is provided. Configurable as a heat pump, a heat engine, or as a combination with one side driving the other, the engine is completely closed, sealed and pressurized with the piston ring as the only internal seal. A miller cycle is created by allowing transfer of the working fluid (typically hydrogen gas) past the piston to balance working fluid pressure only at the extremes of the piston stroke. Two coordinated rotating displacers service opposite sides of one piston. Each displacer manages heat flow, according it its length and shape, through one side of the length of its encasing tube into and out of the working fluid through the other side of the length of its encasing tube. The dead space between the piston and the displacer holds regenerator material.