What caused this?

[YENKO DEUCE REGISTRY]

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if it broke like that on the complete keyway slot, you used a key that was incorrect or too tall, meaning the same thing... LOL

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Hmmmm.... would the balancer slide onto the crank if the key was too 'tall'? I never really thought about it, but....

You might be right Sam, these are old parts so it could just be fatigue. I have your old balancer ready to install - I bought it from Pete after delivering it to him for ya! I must say, I have yet to miss a shift and completely bury the rpm's. I've never even tried to power-shift it, so I still pull my foot off the gas between gears. Ya have draw the line somewhere when it's the original drivetrain, no sense over-revv'ing it It's seen 7500 rpms, but never 9000