Counter balancer key fix

shawn, what did you use for key stock and where did you get it?

I went to the hardware store and bought a piece of Key stock. Cost me a good 80 cents. Between the keystock, and the new lockwashers cost me a total of 4.80 for the fix. The key stock was 1/4 X 1 inch.

Yep Shawn, I agree about the engineering. Make sure you get that bolt on nice and tight. You don't want it going through the side case like it did on mine. My crank is in the shop being replaced with a splined 2001 unit. Maybe I'll trust the beast again after this repair. I did have more luck with the key's I made than Yamaha replacement keys. Good luck.


If someone can take some pics I will put this topic on my this seems to be a common problem with 00' YZ426's! Just email me them @ Later,


Well I made my own key way. Anybody else do this? I made mine about 1/2 again as long as the stock unit, I also made it a press fit into the crank, and had to gently drive the gear on, so its plenty tight. Of all the engineering fopa's on this bike, this has got to be the worst.

Shawn, I did the exact same thing you did, back in 2000. The stock key is 5mm, or about .197". I bought some 5mm keystock from McMaster Carr and it was just a few thou oversize. I simply sanded it down a little bit to get a tight fit. It worked great, and I still have some left if anyone wants a piece.

Scott. I would appreciate the appropriate length of stock from you. My '00 is still going strong and makes no noise indicating a problem with CB, but it would be nice to have a better fix on hand once it DOES start giving me problems. Email me and I'll arrange payment for you. Or, maybe we could wrassle for it.

Could it be that this CB-Key Problem exists also by the YZ 400 '98 models ? My machine makes ugly noises, too !!

Croc, I would check it. the 98's and 99's also have a CB key. They seem to avoid the problem, but I know someone with a 99/420 kit that had the CB problem. Couldn't hurt to check it.


ScottF, What size was the key way you bought to start with? I counldnt find a piece that was .200 thick. So I went with what was closest, .250. I ground it to .200 then sanded it with 320 wet or dry to .198 and pressed it in. I also made mine as long as the slot in the gear short about .020 to I have more keyed area. After getting kinda agro with my press, I thought to myself, why didnt I just freeze that thing and drop it in there, dumb ass. Oh well. It worked, we'll see how long it lasts.


Thinking about this, Im wondering if a piece of 5mm stock frozen and the crank and gear were warmed up to say 200-250 degrees, if you couldnt assemble the unit without reducing the key itself. If so, the interference would be so that "rocking" or slipping would seem almost impossible. I wonder if the original engineering was in that vein, and got changed in production.

Shawn, it was 5mm keystock, about .200". I only had to sand it a few thou. I made it as long as possible for more load bearing area, without interfering with anything. It was probably 50% longer. I made it a light press fit. It sounds like yours was a bit tighter.

Boit, I'll give you a piece free, if I can find it!

I've started to hear this noise louder more recently on mine and I'd like to check the CB key for wear. Does anyone have a concise procedure on how to check/change the keyway? I've looked in here and the ones that go into detail are for changing to an '01 crank and splitting the cases. I'm not that excited yet, I'd like to just try a different key since my stocker lasted 2 yrs. Biggest question is do the cases need to be split to tackle the key replacement or can it be accessed through the clutch cover with the basket removed? I'll check my manual when I get home tonight but I thought some advise from those that had been through this repair before would be useful. Thanks!

It's not hard to check. You'll have to drain the radiators and oil, Reomve the right side case cover. (You don't split the cases.) To do that you'll need to remove the brake and kick-starter and disconnect the water lines to the pump. Then once the cover is off, you should be able to feel play in the cb-drive gear (the inside gear on the crank.) If there is any play, pull the clutch and basket to begin to get to the cb-gear. There is one gear that is bolted on the outside of the cb-gear. Remove it and you'll have easy access for the repair. If you are careful, you should be able to re-use your gaskets and seals. I'm guessing that when you get to the key itself you will find a bit of wear. It don't take much to create a bunch of noise. Make sure you lock-tite those gears when you re-assemble and get them tight. They backed off on me and did a bunch of damage to that right side case. Good luck! Hope this helps. GLEN

After reading these posts, I am a little concerned. The key should never be a press fit or need to be "pounded" into place. This creates a stress on the crank at the keyway and can and will fracture the crank end. I have seen this happen several times where the crank(or balancer shaft) has broken on other models at the keyway because of an improper fit. There is a reason this doesn't come from the factory this way. If you feel the factory fit is too loose, start with an oversize keystock and "massage" till the fit is snug but not tight. Like always, just my opinion. TW

TW massaging a new key-way into place is exactly what I did. When I said it was a press fit, it was just that, I pressed it completely into place with pair of pliers. No pounding required. I believe the main problem with the stock key is that its about 30% too short. Also, they didn't change the balance job when they went to the larger 426 piston from the 400. They just punched it. The 400 motor is smoother than the first year 426 or 2000 model. The 400 motors don't kill the key-way either. So ya gotta wonder about that, why the 426 almost to a bike, kicks the key out and the 400 guys have near zero problem with the identical part. For 01 and up they addressed the problem by using a spline instead of a key-way. I can see how this problem could slip through testing. It would seem like an anomaly, not an obviously under engineered part.

Shawn I appreciate your answer, I am still trying to get familiar with the YZ motors, which this forum has helped immensely. Just trying to relate some experiences I have had with other motors. Thanks again, TW

Thanks guys. I'll probably pick up a new set of gaskets anyway because I'll be attempting this investigation over the holiday weekend and I'd hate to sit out due to a torn gasket.

It was mentioned in another post on the subject to freeze the key and possibly heat the gear to get the best possible finger press fit, do you think this is wise or would this also stress the gear and possibly lead to failure. If so, I can size the key for best fit at room temp on both.

Lastly, I have access to an impact wrench, can that be used to remove the clutch basket nut or is the clutch holding tool needed?

I've used both impact and clutch holders. I think the impact may be the better choice.


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