Retractor pretensioner with piston rest area

A seatbelt tensioner for a seatbelt retractor, comprising, a pipe with a tensioner drive wheel, wherein a pyrotechnical gas generator, a piston, and at least one mass body are arranged in the pipe, wherein the at least one mass body is drivable by a pressure generable by the gas generator and acting on the piston arranged between the gas generator and the at least one mass body, and which can be made to mesh in a force-transmitting manner with the tensioner drive wheel, when exiting the pipe, thus driving a belt shaft of the seatbelt retractor in a tensioning direction that retracts a seatbelt, further comprising,

• the pipe has a receiving section with a first inner diameter, in which the piston having an initial-state outer diameter before installation into the receiving section is arranged in the receiving section prior to an ignition of the gas generator, the pipe has a guide section which adjoins the receiving section and which has a second inner diameter, within which the piston is driven after the ignition of the gas generator, wherein the second inner diameter is smaller than the first inner diameter and smaller than the initial-state outer diameter of the piston.