A seat belt pretensioner mechanism (40) includes a part (36) for attachment to the seat belt (20). A strap (50) is connected to the part (36). A piston (78) and piston rod (58) are movable in a first direction to move the strap (50) and the part (36) to tension the seat belt (20). A pivot connection (59, 62) between the piston rod (58) and the strap (50) causes the strap to move with the piston rod. A cam (70) pivots the strap (50) about the pivot connection (59, 62) and effects movement of the part (36) toward the piston rod (58) as the piston (78) and the piston rod move in the first direction.