A forming unit (20) for forming plastic preforms into plastic bottles, wherein the forming unit comprises a blow mould (20) having a first lateral portion (4), a second lateral portion (6) and a bottom portion (2), wherein said lateral portions as well as the bottom portion (2) form a cavity (15), within which the plastic preforms (10) can be formed into the plastic bottles, wherein the bottom portion (2) can be moved relative to the lateral portions (4) in a closed condition of the blow mould, wherein the forming unit (20) comprises a drive unit in order to move the bottom portion (2) relative to the lateral portions (4, 6), wherein the drive unit preferably has a movable piston element (32) which ist movable along a predefined stroke path. According to the invention, the apparatus has a first stroke path limiting means (22, 24) that limits the stroke path of the bottom portion (2) and/or of the piston element (32) in a first direction, wherein this stroke path limiting means (22, 24) is adjustable in order to modify the stroke path.