A combine threshing system includes a rotor (12) having and in-feed area (54) at a front end of the rotor (12) and a rotor body (60) configured to convey material along a helical path from the front end of the rotor body (60) to a rear end of the rotor body (60). The system also includes a substantially cylindrical threshing chamber (16) positioned circumferentially around and spaced apart from the rotor body (60). The threshing chamber (16) includes a lower portion (504) substantially concentric with the rotor body (60) and having an inner surface (508) positioned a first distance (X) from an outer surface (510) of the rotor body (60) and an upper portion (506/606/706) having a first inner surface (514/614/714) substantially concentric with the rotor body (60) and positioned the first distance (X) from the outer surface (510) and a second inner surface (512/612/712) that is substantially concentric with the rotor body (60), positioned a second distance (Y) from the outer surface (510) and is greater than the first distance (X).
