A rotor top cover assembly for an axial-flow crop processor in a combine harvester is provided. The cover assembly comprises first and second curved cover plate sections at least one of which sections forming an upper part of a generally cylindrical shaped rotor housing for a crop processing rotor. A curved transition plate nests radially inside, and is fastened to, adjoining respective end regions of the cover plate sections. Helical guide vanes are mounted in an axially spaced relationship along an inner surface of at least one of the cover plate sections. At least one guide vane is mounted directly to the transition plate.