Its likely that there is nothing wrong. With no water pump running, and no pressure in the system, you are boiling the water in the heads.
Reading through this thread... It seems you never gave the engine a chance to burb out the air without jumping to conclusions.
I have to agree. My 512 did the exact same thing, twice. Then, I purchased a tool from Snap-On that puts the cooling system into a vacuum (dry, empty, of course) and then "sucks" in the coolant thru a separate hose. I pulled almost 3 gallons into my system. I do have a large, Mark 7 radiator. when I filled the system this way, no getting hot at all. We now fill every new engine build this way and never have an air bubble issue. I bet you just had an air pocket somewhere. I would not run it without a stat and for sure not without belts to turn water pump. I just can't see the heads causing this issue. I think Harbour Freight sells this same tool. Hope this helps.
