5 of 8 allen bolts fixing 2 metal sheets on my car have stripped the hex key.
They are/were 4mm, and I think stainless steel. I've tried drilling one out to
5 and 5.5, but the spirally tool in this pic:

https://www.icloud.com/sharedalbum/#B0zGFssfGG2m5Jn

can't generate enough purchase. It grips a little, but slips. I don't know
what that other bit is in the photo - memory suggests it's a screw extraction
tool I got from Aldi. There isn't enough head on the bolt to use mole grips.

I'm not sure if I'm using the extractor incorrectly, or it's just not very
effective. Thinking now about cutting a slot in the head and trying a flat
blade screwdriver. I don't think they're outrageously tight. I can access the
other side - but I don't want to damage the thread in the sheet the bolt
screws into.

Suggestions appreciated.

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Cheers, Rob