Abstract: A rotary valve for an internal combustion engine is provided in the upper end of a cylinder to control intake and exhaust. A combustion chamber is formed in the valve and the valve is movable co-linearly with respect to the longitudinal axis of the cylinder under the control of a hydraulic fluid whereby the combustion forces on the valve are absorbed by the fluid and the valve can move in response to the pressures and vacuums in the cylinder. A special valve is provided for controlling the flow of the hydraulic fluid, and ducts are provided in the inlet and outlet ports with special seals engaging the rotary valve member.