Internal combustion engine

An internal combustion engine comprising an oil passage, which connects an interior of a crankcase and an interior of a cylinder head, the oil passage being open at an upper section of the crankcase, oil in the cylinder head is transferred to the crankcase via the oil passage and trickles down along an inner wall surface of a skirt of the crankcase to be returned to an oil pan, the crankcase accommodating a crankshaft and an operating member,

• wherein a step is formed on the inner wall surface, the step extending substantially along a projected part, which is a projection of the operating member projected on the inner wall surface in a direction perpendicular to the axis of the crankshaft, such that the step separates at least part of the projected part from a section of the inner wall surface closer to the opening of the oil passage, the step being formed such that the inner wall surface closer to the opening with respect to the step is dented by a greater degree than a part of the step closer to the projected part.