I am not suggestion inside out, but turned upside down. The little piece that looks like a loose rivet should be at the top.
I have found a couple that were installed incorrectly and not trapped in their slot, and work themselves around and wear a deep groove or pit in the cover.
I was thinking that maybe as you were installing the cover, the T-stat may have slipped out of its notch and be tilted, jammed, or turned.
Man it's hard to believe that it will snug down with the thermostat cocked and then
not leak. If it's possible, then it's very easy to do as you can't see whats going on in there. Didn't know about the rivet being at 12:00 either.
When I put it together I ran it with distilled water till the fans cycled a few times. Warmed right up quick to 3 bars BTW. Then drained again, removed radiator etc. Refilled with blue stuff and ran it again. Again warmed right up quick to 3 bars. Fans cycled a couple of times. Let it cool. Topped up radiator and warmed it AGAIN..right up to 3 bars same as ever. Fans only would run for very short time, AT a time. All seemed good. Never occurred to me the Tstat could be in wrong, as I had read SO much about changing them and never a word about that. Also never a word about NEW Tstats being defective. I think
anyone at this point, would call it good.
So back on went everything and here we are.
Oh also the black paint in the hose connections was perfect and not flaking but I removed it inside and out anyway. Gave the radiator a good flushing in the kitchen sink. No leaks at any clamps but snugged them a bit anyway.
It was 10f this morning and I dont like riding in below about 28f. I really want to giver her another chance thinking that MAYBE possibly there was an air pocket in there. Or something. Maybe..
But for now we are in deep freeze with snow and salt everywhere.
But..the brown truck just dropped off my new garage heater!
Tell me Igo, if you really think it's possible that Tstat is cocked in there.