CB mystery knock? hick, anyone


I just bought a 2000yz426, love it. Anyways , i did notice that the bike internals are kind of noisy, i just thought it was the valves and disregarded it.

then i came across a post about the CB key waring out and the noise it made, so i started my bike and listened for it by the clutch cover and much to my chigrin , i heard the noise. Anyways, bike is apart, got the key way out, my question is the key is just slightly rounded at the top, the gear barely had any play in it, maybe a few thousands. I was just curious if this is the cause of the noise with such a minimal parts deteriation. Anyways, thanx for the help

Yes it is. I'd bet next month's pay on it. I've replaced my key twice. Both times the wear looked minimal, but the knock sounded catastrophic. It amazes me everytime I have it apart that so much noise could come from such little wear! Replace it and rock on! For a tighter key than stock you can look into getting some square stock and file it to size. It makes a much tighter fit, however even that is not a permanent fix. If this sounds like an option contact Scott F on this chat for info on getting square stock. I think he gets if from McMaster Carr, but I'm not 100% sure.

Good Luck,


Sounds like Glen’s experience mirrors mine.

I agree that if you can visibly see the rounding on the key then you have definitely found the source of the racket. Even the tiniest amount of observable play is too much when you consider how the counter balancer works.

Whatever play that is available is being taken up EVERY revolution, in both directions (to and fro, to and fro), and the weight and oscillating force/momentum of the CB is meanwhile at work increasing the amount of play every with every revolution.

Eventually bad things will happen to your motor (I speak from experience) if this goes unchecked.

You should congratulate yourself for diagnosing this in time. Now check both keyways, replace the key, and keep an eye (ear?) on it in the future.

If you buy some key stock just make sure it is of the harder variety. A good industrial fastener-supply shop should be able to help you. I think it is like a .75 mm key??? I can’t remember...

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