Does anyone know where i can purchase the 5 special bolts the fit into the rear sprocket and cush drive rubbers?
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MC21R9N track
MC18R2J track
CBR954
CR250

I take it you've tried your Honda dealer?

Have you asked Andy?

If those two fail, there's a place in the UK: http://www.davidsilverspares.co.uk/ who may be able to help.

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

Hi Carl
Carl can you be more precise about this bolt, the bolt that holds sprocket on passes through the alloy carrier into a threaded stud with a hex on that locates in the cushdrive,
I may be able to help with studs not sure about the bolts, let me know I will check for you,
jonmac,

The standard Honda steel sprockets come complete with the special nuts pressed into them. They can have a thin hex nut shape or, just two flats for an open end spanner.
When I fitted the race aluminium sprockets to my bikes I took the special nuts out of the original steel sprockets.
Most of the time they turn in their mountings and pull out easily. Sometimes they need pressing or driving out.
The MC18 is different. It has five special cap head bolts that pass through the sprocket & screw into the aluminium carrier.

You can also use an mc18 sprocket carrier on a 21, use the 18 retaining bolts and you will find them a lot more user friendly, especially if you do track work and find yourself frequently changing sprockets.
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Proud Father of , 05 ktm 400exc supermoto 2018 honda crf rx supermoto

The sprocket bolts are part number 90128-KV3-831, David Silvers part search shows them as discontinued.