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. The second auxiliary gear meshes with a central gear having a center line which coincides with the crankshaft axis. The crank member gear is an internal gear.