The balancer shaft twin gear is like that to stop backlash chattering in the linkage between the crank and the balancer. Without it there would be points in the cycle where the crank is "leading" and other points where the balancer was, eventually you'd be getting wear and noise.
If you've got the engine apart you might as well replace the mains if you can get them cheaply enough - from a bearing factors or the like. If they pulled out with the crank rather than staying in the cases I'd be looking more closely at the crankcases to see if the bearing outers have been turning and wearing the housing. If there's wear you can probably sort it out with loctite.
Like others I've never had to go that far into my XR version of the engine. Top end's been off a few times but I've never had to split the cases.
Last edited by backofbeyond; 18 Jan 2011 at 13:06.
Reason: Inability to write english
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