Abstract: A reciprocating piston mechanism comprises a crankcase and a crankshaft having at least a crankpin The crankshaft is rotatable about a crankshaft axis. The mechanism comprises a crank member which is rotatably mounted on the crankpin, and comprises at least a bearing portion which is eccentrically disposed with respect to the crankpin. The bearing portion has an outer circumferential wall which bears the big end of a connecting rod such that the connecting 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 which meshes with a first auxiliary gear being an external gear. The first auxiliary gear is fixed to a second auxiliary gear via a common auxiliary shaft The auxiliary shaft is mounted to the crankshaft and rotatable with respect thereto about an auxiliary shaft axis which extends parallel to the crankshaft axis.