A seat belt retractor comprises (a) a rotatable spool on which seat belt webbing is wound and (b) a load-limiting system to limit a load applied to a vehicle occupant by the webbing. The load-limiting system includes a flow control orifice through which fluid is forced when a load in excess of a predetermined load is applied to the rotatable spool by the webbing. The cross-sectional flow area of the flow control orifice is adjustable in response to unwinding of webbing from the rotatable spool prior to a vehicle crash event and ceases to be adjustable after the crash event is in progress. The load-limiting system also includes a piston in a cylinder. The piston and cylinder are operatively coupled to the spool such that rotation of the spool causes the piston to move relative to the cylinder, which movement causes fluid to be forced through the flow control orifice.