The rotary valve assembly of the present invention employs a rotary valve body member that is rotatably mounted in a cylindrical opening in a valve casing forming the head above a piston. The axis of this opening and the rotary valve are perpendicular to the axis of the piston and its associated cylinder walls. The intake and exhaust ports are openings that pass through the valve casing from its outer edge to the cylindrical opening containing the rotary valve. There is also a cylinder port between the cylindrical opening in the valve casing and the cylinder containing the piston. The rotary valve body has a circular cutout or scoop portion removed therefrom to allow intake gases to flow from the intake port via this cutout into the cylinder port and then into the cylinder. This same cutout also allows gases to pass from the cylinder port into the exhaust system via the exhaust port. The rotary valve is preferably integral with and driven by a shaft mounted in sealed bearings (mounted in the valve casing) and the shaft is in turn driven mechanically by appropriate coupling to or with the crankshaft. There is no metal-to-metal contact between the rotary valve body and the opening in the head and thus, no need for a separate lubrication supply. The cylindrical opening in the head contains a plurality of non-metallic sealing members which contact the rotary valve body. The rotary valve body has spacers on both sides to prevent metal-to-metal contact with the walls of the cylindrical opening.