The present invention comprises a removable cover plate assembly, which may be quickly attached, detached and adjusted to the exterior of a concave grate of a combine harvester in order to adjust the flow characteristics of the concave or separator grate assemblies. The cover plate assembly improves the threshing capability of the rasp bar threshing cylinder while simultaneously capturing additional threshed grain. Moreover, the cover plate assembly of the present invention enables a single set of concave grate assemblies to better harvest a wider variety of crop types.