Internal combustion engine having concentric camshaft and balance shaft

An internal combustion engine having a horizontally-opposed cylinder arrangement, including at least one pair of opposed pistons, respective cylinders associated with the pistons, and an output shaft, wherein each of the pistons is arranged for reciprocating motion within a respective one of the cylinders, driven by combustion, and wherein the pistons are coupled to the output shaft by a coupling such that said reciprocating motion of the piston drives rotation of the output shaft, wherein the coupling includes a connecting rod coupled to a respective pair of opposed pistons, a slider bearing located for reciprocating movement relative to the connecting rod, the coupling further including a crankshaft that is rotatably mounted within the slider bearing, the engine having a camshaft and a balance shaft wherein the balance shaft is housed in a hollow of the camshaft such that the camshaft and the balance shaft rotate about a common axis.