I have never been able to salvage a stuck dust cap. I either buy a replacement or go with a modern bolt. It won`t be any easier to get it off the next time around even if you can get it off without mangling it.
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My bikes: 1970`s Roberts - 1981 Miyata 912 - 1980`s Ocshner (Chrome) - 1987 Schwinn Circuit - 1987 Schwinn Prologue - 1992 Schwinn Crosspoint - 1999 Schwinn Circuit - 2014 Cannondale Super Six EVO