Since heat of any significance is out of the question, I would recommend the penetrating oil. You may have to let it sit for a number of days and keep reapplying it. You don't need a lot of it, but keep applying it so that capillary action can draw it in to the threads. When this fails, as I suspect it might, I would recommend you drill them out with a left hand thread drill. This will be easier if the heads are not snapped off. Take a ***** punch and strike it in the center of each head so you can center your bit. Use a drill bit smaller in diameter than the threads, about the diameter of the bolt measured from the depth of the thread. Anotherwords, if the bolt diameter is 1/4" then you'll probably want a 3/16" bit. You may well find, that as you are drilling them out, they will spin out on the threads. Hollowing out the bolt, and the heat from the bit, are often enough to do that. I recommend using some light oil, like machine oil or thread cutting oil, as you drill. This will keep you from losing the edge and temper of your bit as you work. Good luck
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Wm. F. Buckley, Jr.