An integrally cast four-stroke engine mono-block (10) includes integrally cast cylinder block (20), cylinder head (40), and portion of a crankcase (30) including crankcase 10 outboard and inboard walls (89, 90). At least parts of outer and inner bearing bosses (21a, 21b) are integrally cast with the cylinder block (20) with the inner bearing boss (21b) integrally cast in the inboard wall (90). At least one cored out longitudinally extending open valve 15 train chamber (88) is disposed between the outboard wall (89) and the cast cylinder block (20).