Just want to add a tip for removing broken studs from aluminum heads/blocks. Worked on military equipment manufacturing and working on expensive CNC aluminum components at present job. Broken screws can cost a lot of money if the affected product needs to be scrapped. You make a funnel out of wax around the culprit bolt, pour muriatic acid into it, covering the bolt.The acid seeps down to the threads only attacking outer portion of the thread, making it easier to remove. The acid will not damage aluminum in a short period, only steel. Even tool steel like taps can be remoed with this method.