Originally Posted by grumpus

You grind it with a mini carbide burr to move each corner outwards - then the next size of hex key doesn't hang up on the corners and can be pounded in, so it engages with the part next to where you ground it out. Or of course you can weld a nut on it. 😁

Originally Posted by Kontact

Hopefully the OP will actually look for hex engagement deeper in the bolt before trying this.

Originally Posted by grumpus

You grind it with a mini carbide burr to move each corner outwards - then the next size of hex key doesn't hang up on the corners and can be pounded in, so it engages with the part next to where you ground it out. Or of course you can weld a nut on it. 😁

Originally Posted by wheelreason

Tacking an allen wrench with the welder is the right answer...

Originally Posted by grumpus

It is an answer, quite a feasible one on the face of it, but it's something of a moot point because it seems that the bolt head doesn't require surgery after all.