Petty much everyone has been there, that gut wrenching moment when you snap off a bolt, or even worse, a tap into your crankcases or some other expensive and hard to replace part. I always had a bear of a time removing things like taps and bolts, maybe I'm just a little ham fisted or maybe I just don't have (and won't pay) for the right tools. Well luckily a little science solves all our problems.
You can dissolve ferrous metals out of non-ferrous metals (such as aluminum, bronze, etc.) with alum (potassium aluminum sulfate). It's a sour tasting pickling spice sold at grocery stores (try your local food co-op) that helps crisp pickled vegetables.
Submerge the offending part into a pyrex dish (you don't want to do this in a steel pot!!!) full of a solution of 4tbsp alum to 1 cup water (as much as you need to completely cover the bolt hole), I then placed the pyrex dish into a pot full of nearly boiling water and let it go. Heat is ESSENTIAL, or the alum won't dissolve. You'll see the steel part start to bubble a little bit (it's letting off pure oxygen) as the alum goes to town.
This isn't the fastest process in the world, it took me about 12 hours before enough of the tap dissolved to get it out. You could probably drill out the center of a bolt to help remove the bulk of the metal and then use the alum to dissolve away the rest leaving perfectly preserved threads. You can also either let it go until the part is completely dissolved, or you can speed things up by breaking little bits off with a sharp pick every hour or so.
The real draw is that there's relatively minimal labor involved, it's really cheap (~$3 per application), totally safe and odorless (let the solution get cold and the alum will form into big crystals you can just save or throw away), and really hard to mess up. Plus if your threads aren't already ruined up, they will be perfectly preserved!
Hope this helps someone out!
