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If the nut is there and stripped, consider a helicoil.
If the internal nut is broken loose / missing there's not much you can do to get it back to "proper" without removing that header....
If proper isn't a total concern, opening the hole up a little and installing a nutsert is likely the easiest choice and it keeps the fastener the correct one.
Going to a self taping bolt the next size up and letting it thread the header.
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Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
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