As stated the bolts for the pre-tensioners are TX50's, how I read it it sounds like you stripped the seats down and then tried to get the tensioner bolt off, your better undoing that 1st, as you can kneel on / or hold & steady the seat if needed, the bolts have lock-thread on them, some come out quite easy & some don't, normally if you feel them tightening up once they're out a little, put WD40 etc... etc... tighten the bolt back up & then undo it again you should be OK (thats next time though), or they can sometimes tighten up & seize.
I don't know haw bad you've butchered it, if it started to undo and then you chewed the bolt you may be able to get some WD40 in the thread (avoiding the bolt head) wind it back in (with mole grips / vise grips) and then undo it again, or grind / dremel the head of trying not to do much damage (covering the seat) get the tensioner off & then you can work on the exposed thread as above with WD40 & grips
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Otherwise you may need a new runner
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