There's no part number shown on the schematics for MC21 model. There is a part number for the MC18 model but it is not the same. And that part number is wrong anyways. i ordered one to see if i could utlitize/modify it in some way. Perhaps the honda sprocket included these and they weren't sold as a separate item? Maybe the part crosses over to other models but without the part number i'm not sure how to find it. Failing that, I'll source through the salvage vendors. Even then, it makes it difficult when i can't identify it without a part number.
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MC21R9N track
MC18R2J track
CBR954
CR250