mine has that pivot pin thing, that is the pivot point for making the head go up and down, that damn pin is constantly working its way out and i have to hammer it every other use. probably protesting being used almost every day.... it also is misaligned but my mom (who had it before me) said it was always like that. Not sure. You'd think Mr Torque the mechanic could look at it for me but........
seriously there are some awesome videos on YT for DIY kitchen aid tuneups.
That pin thingy works out of mine every time I make bread. I don't even think about it anymore, it's almost automatic to check it when I check my bread. That's not the mystery screw, though.