An internal combustion engine comprising a crankcase adapted to rotatably support a crankshaft having an axis, the crankcase defining a plurality of crankcase chambers and including a generally flat manifold mounting surface including therein a first series of inlet openings aligned in the direction of the crankshaft axis, and a second series of inlet openings aligned in the direction of the crankshaft axis, the first and second series being laterally offset with respect to the direction of the crankshaft axis, and each of the inlet openings of the first and second series being respectively adapted to feed air to one of the crankcase chambers and having a minor dimension parallel to the crankshaft axis, a major dimension transverse to the crankshaft axis, and a portion overlapping the inlet openings of the other of the first and second series.