The invention relates to a mandrel locking unit (1) for a rotary printing machine, wherein (1) a mandrel receiving element (9) preferably a bushing receives a bearing (11) in the inner side thereof for receiving a mandrel bearing surface (12) for printing roller mandrels (13) and wherein (1) the mandrel receiving element (9) can be displaced in such a way that the mandrel bearing surface (12) is released by said displacement, and which (1) comprises a printing means cylinder (2) which provides the force required for the displacement and which (2) is provided with a piston which (4) defines the printing area (3) of the printing means cylinder (2) on a boundary surface and which (4) enters into contact with the mandrel receiving element (9) at a point of connection and provides it (9) with the force required for displacement via said point of connection. The inventive mandrel locking unit (1) is characterised in that the distance between the boundary surface and the point of connection is less than the maximum lift of the piston (4) in the printing means cylinder.