A seat belt retractor spool (14) has a seat belt (16) wound on the spool. Ratchet wheels (22) are fixed for rotation with the spool (14). A pawl (30) is movable into a position for blocking rotation of the ratchet wheels (22) and the spool (14) in a belt withdrawal direction (A). A lever (50) is pivotable to move the pawl (30) to the blocking position. A control ring gear assembly (94) is driven upon rotation of the spool (14) to cause the lever (50) to pivot after a predetermined length of the seat belt (16) is withdrawn from the spool (14). The control ring gear assembly (94) includes two members (96 and 118) which are mounted together, and which have projections (114, 122L), respectively, for engaging and pivoting the lever (50). The members (96 and 118) have interlocking portions (104 and 122) which are locked during mounting of the members (96 and 118) for preventing relative movement between the members. The interlocking portions (104 and 122) are unlocked during dismounting of the members to permit repositioning of the members relative to each other, which repositions the projection (122L). Changing the position of the projection (122L) changes the amount of rotation of the ring gear assembly (94) required prior to the projection (122L) pivoting the lever (50) to cause a lock-up of the retractor. This changes the predetermined length of the seat belt (16) that can be withdrawn.