Patent ID: 7380832

Claim:
A belt pretensioner for a vehicle occupant restraint system comprising: a cylinder having an elongate axis; a piston having a first end and a second end mounted in the cylinder for travel therein; a recess in the first end of the piston; an electrically ignitable charge in the recess of the piston; an igniter in the recess of the piston for igniting the ignitable charge; a cable connected to the second end of the piston so that the cable is pulled in the direction of piston travel when the charge is ignited; and axial gas discharge ducts in the piston that are oriented to extend axially substantially parallel to the cylinder elongate axis for directing gas axially from the recess into the cylinder to drive the piston, wherein the piston has a hollow, large diameter portion slightly smaller than the cylinder for sliding therein and in which the charge is held, and the piston has a small diameter portion, and a transition wall between the large and small diameter portions in which the axial gas discharge ducts are formed so that gas is discharged axially into the cylinder from the piston larger diameter portion through the axial ducts and about the piston smaller diameter portion.