Abstract: Apparatus for adjusting the plate segment of an off-set lithographic printing machine drum relative to the drum has first and second blocks, each of which has a threaded bore for receiving the threads of a differential screw. One of the blocks is fastened to the frame of the drum while the other block is moveable on the screw relative to the first block. The threads of the bore in the moveable block have a smaller pitch than the threads in the bore of the fixed block so that the moveable block may be moved in controlled amounts toward and away from the fixed block. The blocks are disposed within a circumferential slot formed on a peripheral edge of the plate segment and as the screw is turned the moveable block acts against an edge of the slot to rotate the plate segment relatively to the drum. A clamp is provided for securing the blocks to the plate segment in adjusted positions.