I'm going to admit to being stupid. It was the starter jammed into the ring gear. I've owned this 28 RPU for 46 yrs, I'm 77, my wife has Alzheimer's, I decided to reduce the collection and sold it. New owner did classic newbie error of failing to retard spark when starting. Brought it back to me to investigate and I got sidetracked by the front oil leak. That was a red herring, the rope seal failed, not a bearing problem. No big deal, the sump had to come off to fix that. Sorry to waste everyone's time.