The comments and suggestions are not meant to disrespect. A thorough diagnosis always starts with the basics. We don't know what anyone has missed. Experienced or not. It should be fairly easy to see if keyway in balancer is matched with keyway on crank. flashlight, mirror, whatever. As I said before if it slipped or sheared key, it would not slip exactly 180 deg. The odds are 360 to 1. Not counting the "minutes". That would be 21,600 to 1. Good luck next weekend.
Be sure and come back and tell us what you find is wrong.
