I am not the mechanic I think I am.
Cleaned the mating surfaces carefully with a scraper, then with a scotch rite pad and mineral spirits, then a clean rag and acetone. Here's the engine side:
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I think I need a new housing:
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Pasted in the thermostat with a little RTV. I'm still not sure it is seated correctly:
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Finished product. I didn't take pics of The Right Stuff bead because it was so messy I am embarrassed. If anything, I used too much:
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Poured my antifreeze mix back in an immediately saw a drip, appears to be coming from the bottom of the housing. I spread some RTV on the bolt threads before installing.
I think it's time for a professional. I suck.
ETA: Thinking maybe it's the bottom bolt. I searched for a new replacement. Discontinued. Damn.