I need a new one of these, the one on my bike looks like someone hammered it to bits. there are only a few teeth left on it.
there isn't a number for it.

Also is the mc28 rear disk the same as the one off the 21?

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The rear disc wont be the same, But it may be the same as a NC30 or another bike with a single swinder, The bit your pointing to is to adjust the chain, there is a pinch bolt onthe top of the swingarm near the back when thats lossened you yous a special tool (like a rear shock spanner) to adjust the chain. think you might have to look on ebay for a complete rear hub, maybe you shoud ask what hubs are compatable, unless someone knows better and you can buy that bit on its own!!!!!!

They get shagged because over the years people lose the propper spanner and end up using a hammer and screwdriver, I must confess i had to do that myself on the vfr8 when i went touring i forgot to bring the spanner.....

PS lt looks like you need part 3 which is the whole unit

Thanks.
part 3 looks expensive

you are right it has the same part number as the nc30

http://akhara.com/nc30/nc30parts/files/32.htm