Brendan, on the original 21 stud engines that I have, there is NO flat washer between the ratchet nut and the pulley. The later 59ab engines that I have do have a flat washer between the ratchet nut and the pulley. I think you could add a washer on the early 21 stud engines but then I'm not sure if you would have enough thread engagement of the ratchet nut into the end of the crank shaft.