Marlin, I believe metal fatigue. Happened to my 61 vette driving down 309 a few years back. Balancer came right off. Split/cracked at the key way. Called Pete, he said not to worry and I got another 283 balancer and put it on. The balancer was probably dampening the vibration in your motor as your monster tach was pushing on 9,000rpm's and cracked from fatigue. Maybe to many missed shifts on a 36 year old balancer.!
