Simple job of changing the thermostat, right? Not today. Soaked with liquid wrench overnight, gave bolt a wack for good measure, first bolt striped my japanese socket set (bad sign). Bought a craftsman set and the 9/16 bolt would not budge. Had to use the breaker bar,...snap. It is now over 100 degrees in the garage and the second bolt felt like the head is also twisting and ready to snap. Stopped work.

Before I break the second bolt, what is the right way to move forward from here. I have not done this before. How do I get the stud out from the first bolt? What should I do with the second bolt?

Apply heat with a propane torch. That should loosen up the corrosion. How much of the bolt that is broken is still sticking up? Do you have an aluminum or cast iron manifold? Heat should do it.

try heating the manifold around the stud and not the stud you are trying to expand the threads in the hole and not the stud. apply heat while trying to turn them with a breaker bar, if they are that bad just go ahead and break them off. remove the housing so you have room to work. try heating them again and apply pressure while grabbing them low on the stud with vice grips, apply pressure and hit the top of the stud with a hammer. i have even gone and welded a nut to the stud and reheated them and removed with a breaker bar. sounds like the studs are soft. use grade 8 studs for maximum strength (look for 5 lines on the head of the stud)if all else fails and you've broken them off, grind them flush to the manifold, center punch them and drill them out, starting with a small bit and working your way up till you see threads on the sides. you can try a easy out-stud extractor but if they are as bad as you say you might just have to drill them and work the thread left in out with a chisel. after you get them out,run a tap into each hole starter tap first then a bottom tap if you have one. use never seize on the new studs when you reinstall them. beleive me your not the first on that has had this problem. i have seen guys break 2 inch studs then after spending 12 hours getting it out they break the 2 inch tap in the same hole.

Thanks for all the advise guys.I think this is going to end up being a drill job. From what I can see on the previous threads it looks like they have been in there for 23 years.

And this was one of those while I am at it type jobs. I changed my leaking shut off valve to the heater core to cool my inside cabin temp, and while I was at it, since I had the coolant drained, I figured I would change the stuck open thermostat.

Sounds like a good reason to up-grade to a new manifold.