An integrally cast four-stroke engine mono-block includes integrally cast cylinder block, cylinder head, and portion of a crankcase including crankcase outboard and inboard walls. At least parts of outer and inner bearing bosses are integrally cast with the cylinder block with the inner bearing boss integrally cast in the inboard wall. At least one cored out longitudinally extending open valve train chamber is disposed between the outboard wall and the cast cylinder block. An alternative embodiment of the integrally cast four-stroke engine mono-block includes at least parts of an outer bearing boss and/or an inner bearing boss integrally cast in the outboard and inboard walls respectively with the cast cylinder block. At least one cored out longitudinally extending open valve train chamber is disposed between the outboard wall and the cast cylinder block. Push tube passages or a belt drive passage may be in the valve train chamber.