If you're talking about a crank dust cover. Start with something called Kroil, PB blser, Liquid Wrench, or a home brew made of 50/50 acetone and auro-transmission fluid. let that soak for 24-48 hours.
Meanwhile use a Dremel, or die grinder to cut a slot almost full width across the cover. Don't worry about scoring the bolt, just don't go deep enough to cut it through.
Now use a super size screwdriver, or you can file or grind a penny to fit the slot and turn the cap out.
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