Abstract: A rotary combine including an elongated rotor assembly having a rotor mounted within a rotor casing for threshing crop materials passing through the rotor assembly. The rotor casing includes an axially elongated threshing section defined by a plurality of concaves that define an arcuate section extending axially along a underside of the rotor casing. Each concave extends in partial circumferential relation relative to the rotor and is supported for movement toward and away from the rotor. An improved concave adjustment mechanism including a powered motor assembly is operably disposed between the frame of the combine and each of the concaves so as to permit on-the-go adjustment of the concaves in unison relative to the rotor to variably control the cooperative threshing relationship between the rotor and the concaves.