More engine knocks from 00426.

Hi all, I'm new to this chat, and looking over some of the topics have given me some insight to the knock that I am hearing in my engine. (My local mechanic didn't have a clue except to say it didn't sound like a crank.) This is what I know, The keeper pin on the balancer drive gear on the crank is worn and needs to be replaced, however with the balancer driven gear removed I can turn the ballancer manually and there is a rough spot in it's movement that produces a knock. The strange thing is it is not consistent with every revolution. Maybe a bearing problem? Also there are metal shavings on the magnets inside my flywheel. More bearing problems? Anybody dealt with this problem yet? Any advice would be appreciated.



The brave taste of death once, but a coward dies many times before his death.

I hope it isn't gear pieces...

My 426 broke its gearbox a couple months ago and Yamaha fixed it.

Take care with your gearbox - is all I can say...


Tranny eh? Good thing Yamaha stood behind their product. Upon further inspection I don't think it was a bearing, just the keeper pin on the balancer drive gear. Thanks for the imput S.M.


That does sound like it could be a tranny problem. I had something similar to this on a YZ 250 last year but I caught it before you could actually hear it knock when it was running. Good luck...


I stopped by a local shop yesterday, and ordered one of those keys (97 cents) - just in case I need one on short notice.


The metal stuck to your flywheel may be debris from the key getting rounded off. But it could also be shavings from the CB bearing.

Of course due to the counterweight the CB will not rotate smoothly but I suppose if it is making noise that isn’t good. Both bearings need to be pressed in, if either needs replacing I think you’ll have to split the cases.

Hope this helps!

Thanks everyone. I hope nothing is wrong with the tranny. Upon further inspection I realized that when I was turning the CB shaft, it was sliding out the other side without my knowledge and making contact with the case. I guess I couldn't believe that such a racket could come from just a worn key. Anyway I have two keys on order and an extra case gasket so I can make a repair on site if necessary. Thanks for all your imput.



If you haven’t yet repaired it and don’t want to wait you can buy a length of straight key from any good automotive supply (I forget what size it is though). Someone else mentioned that Yamaha has two different length keys, this troubles me because if the key is too long you won’t be able to properly tighten the retaining nut (I cut my own key to ride just shorter than flush). Whatever, just make sure the key fits all the way under the CB drive gear.

You can also reuse the gasket successfully if you’re careful.

But be careful with the main gear lock washer. If it is at all mangled get a new one, when this happened to me this washer ended up eating the case cover after colliding with the basket (not much clearance here). That was bad and could’ve been downright disastrous since the bottom end is oiled via a passage in the case cover which feeds the crank.

And don’t forget to line up the marks on the CB and CB drive gear.

Glad to hear there is nothing seriously wrong with your bike.

[This message has been edited by Hick (edited 01-02-2001).]

Thanks for the expertise Hick! I read the message about the keyway a few days ago but have been unable to secure a decent size here in town. I think I can contact McMaster Carr to get one. I haven't tried that yet. Yamaha has a couple coming for me. I think I'll be able to save the lock-washer. It came out clean, but I will make sure I have clearance before I get out and give it hell. I hope to have the parts and time to put it together this weekend.


I bought a length of 5mm keystock for the CB gear repair from McMaster. It was slightly oversize, and had to be sanded down to fit. But that's a good thing, since a tight fit will reduce/eliminate the problem. I can easily cut off a piece and mail it to you. I will even grind it down for a nominal fee. :)

Hi Scott,

I would love some stock! I'm not cheap, but I'd rather do the sanding myself as the fit and wear might be a bit different on my bike. Can I send you a pre-paid envelope? Catch me at



Sure, send me a sase, and I'll cut you off a hunk and send it.

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