The invention relates to a rotor housing assembly for a harvester, having at least one cover plate forming part of a generally cylindrical shaped rotor housing, a central longitudinal axis, and comprising an inner surface, which, when mounted, faces the central longitudinal axis and has a first radius, and one or more rotatable vanes which each are rotatably mounted on an inner surface of the cover plate facing the central longitudinal axis at a rotation point, the one or more rotatable vanes comprising a contact surface with the inner surface, the contact surface having a second radius being larger than the first radius of the cover plate, wherein the inner surface of the top cover plate, per rotatable vane, comprises a symmetrically curved shape such that, during a rotation of the respective vane, the contact surface of the vane with the inner surface of the cover plate substantially follows the shape of the inner surface.