What was supposed to be a simple valve cover gasket replacement, turns out to be a pain. While attempting to remove the valve cover bolts, 3 of them sheared off, at the head. I will have to drill them out. I note that they are composites. Any recommendations as to possible better upgrades? Could simple bolts be adequate? Thanks guys.
Had the same problem on my '89 GT a few years ago - six of the bolts snapped when I had to replace the cam chain tensioner on one of the cylinder banks. I too was considering drilling them out, but was told that it probably wouldn't work since the bolts are much harder than the block. Eventually I found a mechanic who told me that he was fairly certain that he can get them out without damaging the block - but at first wouldn't tell me how. He eventually came to the shop where the car was, and after making sure that no one else was there, he poured what I think was R12 refrigerant (Freon) directly onto the broken bolts. Apparently, the super-low temperature makes the metals shrink - the bolts gets thinner and the hole in the block wider - and after a few applications of the refrigerant, all of them made an audible "tock" as they released. Sure enough, he then pulled all six of them out pretty much with his fingers and a small screwdriver!
So - perhaps you or your mechanic can find something more environmentally friendly than illegally releasing all that Freon in the atmosphere, but the approach certainly does work.
Liquid nitrogen? Or, less dangerously, dry ice chunks?
Crushing the corrosion.
Aluminium oxide is pretty hard and has a crystalline structure - harder than parent aluminium - thus it is quite good at bonding the steel bolt into the aluminium hole.
Aluminium has a higher coefficient of thermal expansion than steel - alu changes size more than the steel with changes in temp.
By cooling both steel bolt and the aluminium hole the alu hole contracts more than the steel bolt does thus making the gap between the steel bolt and the aluminum block smaller. This squashes the crystalline structure of the aluminium oxide thus "breaking it" and releasing the bond.
If yu can cool the steel bolt quickly (try spraying butane gas on it from a lighter refill but don't smoke at the same time) you can break the crystalline structure the other way in tension.