A pretensioning device for use in seat belt retractor assembly, the device comprising: a rotatable spindle configured for taking up seat belt webbing in response to rotation of the spindle, wherein the spindle rotates about a central longitudinal axis; a tread head pinion operative coupled to the spindle, the tread head pinion configured to rotate in response to a pretensioning load applied thereto, wherein rotation of the tread head rotates the spindle; a deformable rod configured to translate along a predetermined path into direct engagement with the tread head pinion in response to actuation of the rod, wherein the translation and engagement of the rod with the tread head pinion rotates the tread head pinion; wherein the tread head pinion is formed as a single piece and comprises: a first side facing away from the spindle and a second side facing toward the spindle; a ring portion extending between the first and second sides (70,72) and having a plurality of radially protruding teeth for receiving the pretensioning load from the rod; a pair of flanges including a first flange at the first side and a second flange at the second side disposed on opposite sides of the teeth and projecting radially outward beyond the teeth, the flanges disposed longitudinally adjacent the plurality of teeth, wherein the teeth blend into the flanges to define a cavity between adjacent teeth and the flanges; and a bearing portion projecting axially from the second flange toward the spindle, the bearing portion defining a bearing surface against which the spindle will bear. a rotatable spindle configured for taking up seat belt webbing in response to rotation of the spindle, wherein the spindle rotates about a central longitudinal axis; a tread head pinion operative coupled to the spindle, the tread head pinion configured to rotate in response to a pretensioning load applied thereto, wherein rotation of the tread head rotates the spindle; a deformable rod configured to translate along a predetermined path into direct engagement with the tread head pinion in response to actuation of the rod, wherein the translation and engagement of the rod with the tread head pinion rotates the tread head pinion; wherein the tread head pinion is formed as a single piece and comprises: a first side facing away from the spindle and a second side facing toward the spindle; a ring portion extending between the first and second sides (70,72) and having a plurality of radially protruding teeth for receiving the pretensioning load from the rod; a pair of flanges including a first flange at the first side and a second flange at the second side disposed on opposite sides of the teeth and projecting radially outward beyond the teeth, the flanges disposed longitudinally adjacent the plurality of teeth, wherein the teeth blend into the flanges to define a cavity between adjacent teeth and the flanges; and a bearing portion projecting axially from the second flange toward the spindle, the bearing portion defining a bearing surface against which the spindle will bear. a first side facing away from the spindle and a second side facing toward the spindle; a ring portion extending between the first and second sides (70,72) and having a plurality of radially protruding teeth for receiving the pretensioning load from the rod; a pair of flanges including a first flange at the first side and a second flange at the second side disposed on opposite sides of the teeth and projecting radially outward beyond the teeth, the flanges disposed longitudinally adjacent the plurality of teeth, wherein the teeth blend into the flanges to define a cavity between adjacent teeth and the flanges; and a bearing portion projecting axially from the second flange toward the spindle, the bearing portion defining a bearing surface against which the spindle will bear.