An axial flow combine having a crop harvesting header including a feeder for cutting a crop and feeding it rearwardly into a threshing and separating mechanism. The mechanism includes a casing for receiving the crop material and a rotor within the casing for threshing and separating grain from the crop material. A plurality of crop material directing vanes is disposed within the casing and cooperate with the rotor in directing the material generally helically along the casing. Means are included for selectively and simultaneously adjusting the position of the vanes to change the rate of movement of the crop material through the casing.