Combine harvester

A combine harvester including a fixed structure, a threshing unit carried by said fixed structure, a pair of transverse contra-rotating distributor augers rotatably mounted in said fixed structure and located downstream of said threshing unit relative to the product flow direction and arranged to receive the product leaving said threshing unit and discharge it downwardly for distribution over the entire width of the harvester, an accelerator roller rotatably mounted in said fixed structure below said two distributor augers for accelerating the fall of the product, a box element having two opposing longitudinal walls adjustably secured to said fixed structure adjacent said accelerator roller so as to selectively present one of said walls to said accelerator roller to provide a substantially vertical fixed wall adjacent the accelerator roller for cooperation therewith to accelerate the product, and a cleaning unit located below said accelerator roller and including at least one fan for directing a current of air at the product accelerated by said accelerator roller wherein one of said walls of said box element is provided with a rough hulling surface and the other of said walls of said box element is provided with a smooth surface.