Abstract: A pretensioner for a seat belt system has first and second piston-cylinder arrangements arranged generally one on each side of a spool shaft so that in travelling along the respective cylinder, each piston engages the periphery of the spool shaft and causes it to rotate. A sensor is responsive to a crash situation to detonate a first piston drive means, and means for detonating a drive means for the second piston at the end of the stroke of the first piston. The spool shaft may be a component of a seat belt retractor. Alternatively, the spool shaft is wound with a cable that pulls on a seat belt or seat belt buckle.