I'm trying to get a couple of allen bolts out of the seat tube of my bike so that I can fit a rack.They don't want to come out though. The allen key sockets are rounded off, and they seem pretty well corroded on. I've tried dripping oil where the sides meet. I've also taken a hack saw to one of them and cut a groove across so I could use a flat-blade screw driver on it, but still no joy. Any ideas anyone?