http://www.bikepics.com/pictures/1926511/http://www.bikepics.com/pictures/1926529/hi geeza ,you can see the difference between the two models , the 18 screws into the carrier and the 21 bolts onto it . The 21 sprockets need a bigger hole for the bolt to go through . You need to clean out those holes on your sprocket so the nuts don't get jammed .