Two parts of a seat belt retractor spool sub-assembly are fixed together in an arrangement with plastically deformable formations on a first ratchet wheel and one end of a spool and corresponding deforming formations on a second ratchet wheel and said one end of the spool. A spool for a seat belt retractor is assembled with a ratchet wheel by: mounting a retractor frame on an assembly jig which has a spring loaded stop; mounting a spool in the frame by aligning the spool axis with the spring loaded stop; and pushing a ratchet wheel onto the end of the spool opposite to that contacting the stop until the ratchet wheel abuts the frame.