Extracting a rounded off internal hex key csk bolt

You could try this .
Re grind a 8mm drill backwards so the sharp edge works like a LH drill . Drill a 6mm hole about 5mm deep in the socket .Then use the reground drill. Put your electric drill in reverse and it might give enough bite to unscrew it. If not do as MB says and drill the head off and there will be enough thread left to get some grips off it when the tank is off.

I tried a version of this once and went from having a sheared valve cover bolt to having a sheared valve cover bolt with a broken HSS drill bit in the middle of it :blast
 


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