Drill a smaller hole into the center of the broken off bolt, and then use the appropriate size "easy-out" (google: easy-out bolt remover).
If that doesn't work, drill out the whole bolt (exactly centered) without disturbing the threads. Try to pick out the bolt left in some of the top threads, and then re-tap it with the appropriate size tap. Used oil on the tap.
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Prince Edward Island
'64 Parisienne CS "barn find" - last on the road in '86 ... Owner Protection Plan booklet, original paint, original near-mint aqua interior, original aqua GM floor mats, original 283, factory posi, and original rust.
Sometimes if it's flush if you can find a guy who can weld good, you can set a nut on there and have him weld it to the bolt. The combination of the nut welded on and the heat may loosen it enough to turn the nut and extract the broken part. May be worth checking into. Much nicer than drilling if it works.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars
mig , either building it up , or welding a nut over the stud.... crank the heat up , snap the vise grips on to quench the heat out of it ! let it cool slightly so it shrinks ,,,,,and out it comes.
I use this trick with aluminum parts that have steel bolts broken in them all the time....
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later...rog
AADD supporting member !!
I'm a collector...not a builder!!Located in sunny central Saskatchewan at the lakehead!
give this a try: just use a narrow punch and a small hammer, tap the outside area of the bolt, and see if you can turn it counter clockwise. As there's adequate oil in the area, the bolt is unlikely to be rusted, and may be easy to turn out. I did one on my 454 about 30 years ago, it was actually quite easy to get out.
When you change the covers, you can remove it then C2