As some of you figured out for yesterday’s challenge the balance wheel was upside down.
At this angle you can see how the balance wheel contacts the fourth wheel.
Once I restaffed the balance (right-side up) I had to adjust the rate by 86 seconds to get it within Rolex specifications. That is a lot of turns of the Microstella screws, but there was plenty of room since the previous repairer had turned the large screws all the way in to make up for some kind of lost energy. What could have caused that?