RE: Small, broken drills in aluminum...
For this one I put my professional hat on (chemist. biochemist)-what you need is 30% nitric acid, which is passive towards aluminium, but will dissolve the steel drill bit (or broken tap, or broken screw as the case may be) Immerse the plate in 30% nitric (use a plastic bowl or a ceramic or glass dish. Use rubber gloves when handling the stuff. [30% v/v is not especially strong, but stronger than dilute-it works out about 2.25 X dilution of the normal concentrated stuff, which is 67%] There will not be any significant fuming, merely slight bubbling around the site of the hole. The process will take a while-several hours to overnight-since thee is only a small access area for the acid. Eventually the bit will either be completely eaten away, or reduced in size so that it simply falls out.
I've used this trick on both engines and alloy pans on a number of occasions.
Use ONLY nitric acid-you cannot use the other common mineral acids for this process.
'ffkiwi'