The invention consists of a reversible mechanism in which swingable arms, arranged in pairs, and actuated by the cams of a rotary camshaft, serve to actuate valve-operating members, which are so disposed that they can be brought into engagement with either arm of the corresponding pair of arms, characterised in this, that the said valve-operating members are operatively connected with a common reversing shaft, by means of which they can simultaneously be brought into engagement with either arm of the respective pair of arms.