Stuck Thermostat Housing Series II

[willincalgary]

I am trying to change my '59 Series II from the Series II thermostat housing/thermostat to the IIa-III version. I was having all sorts of overheating issues, running very hot overall and suspect from the behaviour of the temperature gauge on cold days that the old thermostat was stuck partially open. (new radiator, hoses, good low mileage water pump) Two of the three bolts twisted off while trying to remove the old housing and it still will not budge. Tried penetrating oil on the stuck bolts and then some not so gentle persuasion with a fine wedge and a hammer trying to lift the housing from its lower face, still no movement. Don't really care about the housing itself, as I'm trying to replace it, so I drilled out 1/2 the length of the offending bolts to remove some binding with the bolt's shank and hit it with more penetrating oil. Still nothing. I am hessitant to drill out the bolt down to the top of the threads as I would like to have something to grab onto to try to coax the bolts out of the head when I get the housing off. Any good ideas for removing the housing? Heat? a bigger hammer?
Any help would be appreciated.

[I Leak Oil]

Chances are good the bolts will have to be drilled out anyway so I'd vote for continuing to drill them the rest of the way. Leave maybe 1/8" or so at the bottom. Or try an air chesel to split the housing.
Jason T.

[Tim Smith]

This is one of those situations where I'd probably end up using too much heat and melting something unexpected. Although, I'm sure I'd get the housing off in the end.

[singingcamel]

Tim
Sometime you just have to go for it.

[willincalgary]

The saga....

Well, that was a battle. Drilled the bolts out and managed to get the old housing off with a swift crack of a chisel (you were right Jason T, no chance without drilling them out). Ground the remaing bolt off flat and drilled into the bolts to use an extractor . Got one out with the extractor and broke it in the other bolt . Bought solid carbide drill bits (a bit expensive but truly phenomenal; drilled the extractor easier than my cheap bits drilled the bolts!) and removed the extractor . I then unsuccessfully tried a second extractor and gave up on that approach. Elected to drill out and retap the hole. Bought bottoming tap to finish off the threads in the blind hole. I am now thinking of replacing the bolts with studs lest I ever have to go through this again. Anybody think of any reason why not to?

[I Leak Oil]

No need to use studs unless you really want to. If you use studs or bolts use anti-seize on the entire bolt. That will keep the threads and the shoulder of the bolt from rusting and swelling like you just had the pleasure of experiencing!
Jason T.

[willincalgary]

Well. The retapping the drilled out bolt didn't work as I probably wasn't near enough the center of the original bolt with my pilot hole. Anyhow a helicoil fixed that problem and Grover is running perfectly with a new thermostat and a Kenlowe cooling fan. Thanks for your help.