Abstract: Interchangeable cassettes containing a blanket cylinder and a plate cylinder of varying circumferences can be located in a printing tower of an offset printing apparatus. An impression roller is journaled to a pair of sub-frame arms pivoted to the frame by means of a pivot shaft which extends through the gear side frame wall. The impression roller shaft extends through the gear side frame wall in an arcuate slot, and the impression gear meshes with the pivot shaft gear at all angles of displacement of the impression gear. A pivoting sub-frame includes exterior plates mounted to the pivot shaft and on the exterior of the frame walls. Hydraulic clamping devices can clamp the exterior sub-frame plates and the sub-frame arms to the frame wall.