We've replaced both parts on an 8780, the worm and the notched part. Worm was thin and the notched was starting to round. Started dropping when thrashing wheat. Once changed never happened again. It's not that bad of a job to do.
8780xp concave drops
I have an 8780xp with the manual concave adjustment. I am currently combining flax and when I hit a slug or hear the rotor rumble the concave adjustment gear jumps the worm gear and then the concave opens up. I can't seem to get it to stop. Concave clearance is set to 1/8". The worm gear and concave gear are now starting to show wear due to the jumping. Just can't figure out how it jumps as when mated the gears are tight. If anyone has any advice I would appreciate it. Its making harvest frustrating 
