What caused this?

[YENKO DEUCE REGISTRY]

I love that pic, we spent this past Sunday afternoon hand-sanding the rear diff. Can't beat the father-son time!

Kim,
Yes, the bolt was very loose when I limped it into the trailer at SCR9. Pete Gerling and I started looking at it, and noticed the slop in the belt. I unscrewed the bolt with my fingers, and the balancer nearly fell off. The key fell out onto the floor of the trailer. The key initially fit nice and tight when I installed it, but it was mangled pretty bad after the break. This was the original balancer for this engine, and fit very tight when installed. The replacement balancer also fits very tight - but I can only get it installed for about an inch. I need to finish cleaning up the keyway area with a small file, then I can install it completely.

I'm open to any advice!