Honda CBR F3 shock will work but you need one with ride hieght adjustment. I have a Fox CBRF3 on the mc-21. It will also fit on a MC-18 but it is about 1 inch longer than stock with the ride heigth adjuster all the way in, ok with a 17 inch rim. I did not fit the mc-21 with the CBR shock so I don't know what mods are needed. On the mc-18 the upper bushings needed to be norrowed and the hole need to be opened up.
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