Broken thermostat housing bolt

Head of one of the bolts on my thermostat housing twisted off when I tried to remove it.  I've been soaking what's left with PB Blaster, but I doubt that an Ez-out is going to even touch it.  I guess that means I'm going to have to drill it out.  Any ideas for how to go about this without damaging the threads in the intake manifold?  It's the factory steel intake.

AMC RICK 401 wrote: Is there any of the bolt sticking up above the manifold? you can weld a nut on it and the heat will usually help it to come out. Try that before drilling

i had one break off last year and thought i was out of luck... i took a propane torch and heated up the intake around it and was able to back it out since it had a little bit of material left. I can't remember how I grabbed it - i might have even used a dremel to cut a slot the seat a screwdriver in and back it out.

AMC RICK 401 wrote: Is there any of the bolt sticking up above the manifold? you can weld a nut on it and the heat will usually help it to come out. Try that before drilling

X2 what Rick said!! Been there done that more than once!   

Bolt the thermostat housing on and use it as a drill guide with a bushing around the drill bit.  The outer diameter of the bushing should be equal to the inner diameter of the eye on the housing. The inner diameter of the bushing should be equal to the drill diameter. This will keep the drill bit centred and square on the busted bolt.

You will want to start with a small bit and work your way up to the minor diameter of the bolt. That will leave all the threads in the intake intact, and you can chase them with a bottoming tap. Just be sure to take a depth measurement and mark the drill bit so you don't drill too deep. Even if you do damage the threads, you can still go a little oversize and install a Heli-coil.

I used this method on a busted exhaust manifold bolt (SBC) last summer. Worked like a charm, and I never had to pull the head.