I have been riding my RD-5800 this week with a slightly misaligned hanger. It had some difficult shifting, but it was ridable.
So the hanger alignment tool arrived today, and I decided to clean the derailleur and re-grease the jockey wheels after aligning the hanger. To my incredible surprise, when trying to re-tighten the tension pulley screw, the aluminium forming the thread broke off from the inner plate. I have now a useless derailleur.
I am trying to establish the probable cause of the problem to prevent a similar scenario in the future. Firstly, I would like to precise that the bolt has never been subjected to more than 4 N.m (Shimano max = 5 N.m), so I don't think that is the problem.
Every 2-3 weeks, I remove the jockey wheels to re-grease the bushings. I have been doing that for the last 8 months.
Do you think that the frequent removal of the jockey wheels caused the problem, or it may be due to the vibration experienced by the misaligned hanger?
Follow-up: I have temporarily replaced the bolt with a M5 longer one and a nut on the other side. It is not the better solution, since it is threaded in the bushing portion, but it will last until my new derailleur arrives.