Penetrating oil. Soak it down. Make a flat spot on the end of the stud and center punch it. Drill a small, 1/8" or so hole all the way through it.On second thought if this bolt goes down into the water jacket, don't drill all the way through. If the bolt hole has a bottom, drill all the way through it and fill with penetrating oil. Then use an "easy out" to remove the bolt. Make a bigger hole for the easy out. The really small easy outs can break and they are very hard to remove if you break them.