Tap the stripped 6mm with 1/4-28; it's a better fit than 1/4-20. After running the tap all the way in, switch to a "bottoming" tap, to take full advantage of the depth of the threads in the hole. Tip: to make a bottoming tap out of a standard tap, grind the end down until there are only one or two tapered threads on the end.
(and please change your avatar! You're creeping me out... If I want to see a half-naked, ugly, old bald guy, I can always look in the bathroom mirror
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