A combine threshing system includes a rotor having and in-feed area at a front end of the rotor and a rotor body configured to convey material along a helical path from the front end of the rotor body to a rear end of the rotor body. The system also includes a substantially cylindrical threshing chamber positioned circumferentially around and spaced apart from the rotor body. The threshing chamber includes a lower portion substantially concentric with the rotor body and having an inner surface positioned a first distance from an outer surface of the rotor body and an upper portion having a first inner surface substantially concentric with the rotor body and positioned the first distance from the outer surface and a second inner surface that is substantially concentric with the rotor body, positioned a second distance from the outer surface and is greater than the first distance.
