a valve operating mechanism is described for an internal combustion engine in which at least one valve of an engine cylinder is operated by two cams . the cams have respective cam followers which are resiliently biased to remain in contact with the cams at all times and which act on the valve by way of a summation linkage in such a manner that the displacement of the valve at any instant is determined by a combination of the displacements of the two cam followers . in the invention , movement of each of the two cam followers is transmitted to the summation linkage by way of a respective one of two pushrods .