I've got an old Saint rear derailer (RD-M800) that I'd like to adapt for use with horizontal dropouts. I have all of the parts I need, but I can't seem to remove the current bracket (Part #5 here: https://techdocs.shimano.com/media/te...9830609155.pdf). The diagram shows a simple threaded bolt, with a clip to keep the spring and other innards in place- it seems like I should simply be able to turn it counterclockwise, but I've tried quite a bit of torque on it and it hasn't broken loose- I'd rather not break a more-or-less irreplaceable bolt, and it's only a 4mm Allen head. Has anyone performed the operation? Any tips?

Penetrating oil (Kroil is the best), time to soak and moderate heat. I am not familiar with the part, are you certain that it is right-hand threaded?

Penetrating oil is a good idea. Right hand thread is an assumption (check the exploded diagram above, the bolt is part #1, the bracket is #5). I don't know what metals the two parts are made of.