Re: head bolt broke off!
1st - what engine are we dealing with here?
If you have the other bolts off then grab a block of wood and a ~5lb short handle sledge and start whacking away laterally on the head away from the bolt.
Your goal here is to rotate the head. Your favorite penetrating oil might help on the bolt to head "stiction", although it's not necessary.
As Daviet has said heat will help but is usually not needed. This helps to avoid warping the head.
You can also use wood chisels. Flat side MUST face the block. If you can, start away from the bolt. This will help break the gasket seal, if there is one. It'll be a little easier to rotate.
On the bigger motors there is ~1/4" of seal surface...so don't be a pig with any individual chisel. Wider chisels help to spread the load, but is not always accessible.
Now...Your broken bolt. The bolt gods probably did you a favor breaking it. The portion sticking out from the block makes it a much easier job and don't even dream about an easy-out.
You will, however, need heat and good vice grips at a minimum. Welding a nut onto the stud is ideal, but you may not have a welder. The slightly round vice grip jaws tend to work best. You then need a hot torch - acetylene or a good mapp gas - so the block can be heated fairly quickly.
Some like to use a candle on the exposed stud, but most do it dry. Once you get the localized block area hot, you're looking to work the stud back and forth without breaking it off.
Unfortunately, it does take some experience sizing it up and having a feel for the torque. Also getting the aluminum hot without melting it - however this would take a fair amount of time with map gas.
If the aluminum threads come out with the stud, just drill and tap using a heli-coil or timesert.
It might be worth a few phone calls to have the stud removed for you. It isn't pricey.
As boobie said, SOLID cobalt alloy drill bits are the way to go. They deal well with heat and can be re-sharped. They are usually available in 5% and 8%, M36 and M42. The m42 are more brittle, so I'd avoid them in smaller sizes (my opinion).
Titanium bits are simply a coating, don't like heat and you'll remove the coating when re-sharpened, but certainly an improvement over the steel base. There are also cobalt coated bits and these are - again - an improvement over the steel base.
I think that's enough for now.
