I chiselled them round to unscrew them. As the bolts turned and the chisel marks were all around the head it made it a little easier. On an M8 thread it's only really practical to use a 4mm stud extractor as the core of the bolt is 6mm that leave a 1mm wall. Any less on the wall thinkness and the stud extractor would expand the wall and actually tighten the screw in the hole
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