An internal combustion engine for a motorcycle, comprising a crankshaft extending transversely of the motorcycle and having at least two crankpin portions, power cylinders consisting of a first bank of cylinders inclined forwardly from a vertical plane which passes through the axis of rotation of the crankshaft and a second bank of cylinders inclined rearwardly from the plane, the angle between the first and second banks of cylinders being less than 90 degrees about the axis of rotation of the crankshaft. One of the cylinders of the first bank and one of the cylinders of the second bank are associated with one of the crankpin portions and another cylinder of the first bank and another cylinder of the second bank are associated with the other crankpin portion. At least two camshafts are provided one for each of the aforesaid banks and are driven by drive means arranged in laterally middle portions of the engine casing structure. A balancer assembly is positioned forward of the crankshaft and driven for rotation about an axis parallel with the crankshaft.