Abstract: An engine block is formed by a single piece lower crankcase and a single piece upper crankcase having abutting surfaces sealed with a gasket. The abutting surfaces have semicircular notches aligned with one another to form a circular opening for a crankshaft. The upper crankcase includes central section from which extends a barrel unit that contains the entire cylinder bore for a piston. An integral housing for the cylinder valves and rocker arms is formed on the barrel unit and an integral intake manifold extends from the housing. Forming the engine block from two primary components reduces the number of joints that require machining and gaskets.