Bob. You hit the nail on the head. But. Underneath the frame right between the collar with the arrow and the clutch , is a hole in the body of the machine. There is a screw in the hole that holds the race/collar in place. That screw was loose. Jammed everything west and tightened it up. Also. The stitch length adjustment wheel had shifted east a tad also. Loosened those 2 screws and tapped it over about 3/16 west and the pin now grabs and the stitch length is solid now.
This place is great. Your direction was enough to guide me in the right direction. I now see there is a flat spot on the shaft and the collar with the arrow can’t rotate unless you loosen the screw way off.
I’m sewing now. But I still love my 5100 more. Lol.