Abstract: To facilitate construction of internal combustion engines rotating in opposite directions using the same crankcase, three recessed bearings 5, 5a, 5b, are formed in the crankcase. For engines operating in a first direction of rotation, a first bearing 5 supports a first intermediate gear 4 to provide the driving connection between the crankshaft 2 and the camshaft 7, and for an engine having an opposite direction of rotation the second and third bearings 5a, 5b, support a second intermediate gear 4 and the first intermediate gear 8 to provide the driving connection between the crankshaft 2 and the camshaft 7. This construction permits a simple conversion operation to change the rotation direction of the internal combustion engine.