A threshing and separating mechanism for a combine includes an elongated rotor mounted for rotation about a rotor axis on the combine within a rotor housing. The rotor has a threshing portion and a separating portion. The rotor has a rotor tube that is tapered continuously in the threshing portion and has threshing elements extending from the rotor tube. The housing has a threshing section and a separating section corresponding to the threshing portion and the separating portion. The housing surrounds the rotor and is spaced from the rotor tube to form an annular space between the rotor tube and the housing for crop material to flow through in an axial crop flow direction from an inlet end of the housing to an outlet end of the housing. The housing includes a smooth transition section between the threshing section and the separating section of the housing.
