A snug-fitting woodruff key is an absolute necessity. Even with a bolt holding the balancer tight against the timing gear, a sloppy fit between the balancer and the key will unscrew the bolt. At 2000 RPM the balancer will be trying to move back and forth 66 times a second due relative motion between the balancer and the key. A worn key, and/or extra clearance between the slot and the key will make it impossible for a center bolt to stay tight. "No exceptions- - - - -no excuses!"
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!