A rocking mechanism, through which linear reciprocating movement of a piston in a cylinder of an internal combustion engine can be transferred, from the remote end of a con rod on which the piston is mounted to a crankshaft, includes an oscillating device and a mounting device by which the oscillating device is mountable on a crankcase of the engine for reversible oscillating movement of the oscillating device on a first axis parallel to the crankshaft axis. The oscillating device is adapted for pivotally connecting to the little end of a further con rod having a big end connectable to the crankshaft for relative rotation, between the rocking mechanism and the further con rod, on a second axis offset from and parallel to the first axis. The oscillating device also is adapted for pivotally connecting to the remote end of the con rod on which the piston is mounted for relative rotation, between the rocking mechanism and said remote end, on a third axis offset from and parallel to each of the first and second axes. The first and third axes are substantially equidistant from the second axis and the oscillating device is pivotally connectable to the respective con rods so that the first axis and the crankshaft axis are positioned to opposite sides of an axis on which the piston is reversibly reciprocable.