Safety belt pretensioner having a pyrotechnical piston/cylinder drive

A safety belt pretensioner having a frame and a pyrotechnical piston/cylinder drive which comprises a cylinder, a piston accommodated in said cylinder, a gas generator and a base pivotally mounted on said frame, said cylinder, piston and base forming a vehicle-sensitive inertial mass and a cable being connected to said piston and extending out of said base, wherein said piston is received in said cylinder with radial clearance and wherein said piston has an annular groove around its periphery and said groove has a bottom shaped to act as a ramp extending between a first bottom end of smaller diameter and a second bottom end of larger diameter, a deformable annular seal being located in said groove adjacent said first bottom end without contacting said cylinder, and said annular seal being adapted to move along said groove bottom towards said second bottom end and radially outwardly into contact with said cylinder under the action of pressurized gas supplied by said gas generator.