Originally Posted by
Crankycrank
So far you've done a pretty impressive job of drilling right down the center of the bolt but I would try a smaller diameter bit first to drill all the way through then, if needed, increase the bit size. You might get lucky with the smaller or larger bit, and it will catch and spin the bolt out. As mentioned already, first use some good penetrant like Kroil or PB blaster and soak for a day. You can also use a sheet metal screw as an "Easy Out" type of tool since you don't need reverse turning to remove that bolt. If you have some good vice grip pliers you may still be able to grab that stump and remove it but clamp it on as hard as you can.
+1 to this..
the tapered shape of the "stub" will, most likely, mean the vise grip idea is not going to work.
someone suggested grinding a slot/ use screwdriver method... won't work on a screw that's so stuck in the aluminum that it broke, but Does Work for a bolt that broke from over-tightening!
i like using left hand/reverse direction drills on blind hole broken bolts/screws... the drilling process creates Heat that can break loose a stuck bolt or screw.
one thing though... ANY lube on the screw, WHEN DRILLING, will have the drill not penetrating the broken screw, and that will then cause advanced wear of the drill as it work hardens the screw material.
clean off any lube Before Drilling.
Excess RPM can and will work harden the surface, and also Rapidly Dulls the Drill Bit.
people then tend to press harder, which then snaps the small drill bit...