I have also done this mod to my mc21. i have used the rs250gp caliper and bracket along with a torsion bar that needed a mount welded on the inside of the s/arm, to be honest i dont know why i bothered, as i never use the rear brake anyway, it's been there 5 years and the pads are still brand new, looks trick though, but nobody ever notices it.
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Proud Father of , 05 ktm 400exc supermoto 2018 honda crf rx supermoto