Abstract: An improved double-acting, nonexpansion, noncondensing, piston steam engine. A coiled tube flash boiler enclosed in an insulated fire chamber is located above and contiguous with the cylinder head of the steam engine. The base of the fire chamber has a cylindrical opening to enable the heat from the flames in the fire chamber to be transferred directly to the top cylinder head for enhanced heat transfer to the cylinder. Two parallel crank shafts, each having a spur gear fixed thereon which meshes with the spur gear fixed on the other, are mounted on pillow block support bearings. A "T" linkage interconnects the crank shafts and the piston of the steam engine. Two rotary valves are provided to control the flow of high temperature steam and spent steam to and from the cylinder.