After Martket Thermostat Housing

Dana67Dart said

I used their standard gasket (NO SEALER and inch pounds, 12inch pounds IIRC)

Did you clean out the holes the bolts go into?

Thermostat Housing Gasket - Silicone Gaskets - Real Gaskets Tennessee

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Spotless. I also coated the bolts with RTV the second time around with the idea maybe the head had a hairline crack around a bolt hole. No dice.