A reciprocating piston mechanism comprises a crankcase and a crankshaft (rotatable about a crankshaft axis) which has at least a crankpin having a crankpin. A connecting rod includes a big end rotatably mounted on the crankpin and a small end rotatably connected to a piston. The crank member is provided with a crank member gear that meshes with an intermediate gear, both gears of which are external gears along with first and second auxiliary gears. The intermediate gear meshes with the first auxiliary gear. The first auxiliary gear is fixed to the second auxiliary gear via a common auxiliary shaft that mounts to the crankshaft and rotates with respect thereto about an auxiliary shaft axis that extends parallel to the crankshaft axis. The second auxiliary gear meshes with a central gear. The central gear is an internal gear, which has a center line that coincides with the crankshaft axis.