Abstract: A reciprocating piston mechanism comprises a crankcase and a crankshaft. The crankshaft is supported by the crankcase and rotatable with respect thereto about a crankshaft axis. The mechanism further comprises at least a connecting rod including a big end and a small end, a piston which is rotatably connected to the small end, and a crank member which is rotatably mounted on the crankpin. The crank member comprises at least a bearing portion and has an outer circumferential wall which bears the big end of the rod such that the rod is rotatably mounted on the bearing portion of the crank member via the big end. The crank member is provided with a crank member gear. The crank member gear meshes with at least an intermediate gear, which also meshes with an auxiliary gear. The auxiliary gear is fixed to an auxiliary shaft that extends concentrically through the crankshaft.