Since the Mustang engine slopes downward at a good angle, unless you have the rear end on jack stands when filling, the vented thermostat will allow virtually all the air to escape. The only trapped air would then be at the high spots in the heater hoses, maybe a cup, certainly nowhere near a half gallon.

 

I hate to say I told you so... (if this ends up being the issue in the end)

It usually always pays to totally eliminate the simple stuff first, I know because way more than once I have done a lot of work to find that I had overlooked a simple fix.

Worst case is that it IS the thermostat, but damage has been done already.... I have seen this more than once.... and it sucks.

As far as your question about the lower hose getting hot, it makes sense to me that this would happen.

 

A cup, really ? Then you can just fill the radiator and cap it and call it good ?

 

This a prime example of overthinking a simple overheating problem. I was so tied up in what could be failing i didn't bother to check what was most likely the cause. What a moron.

I never let the block see over 218deg, so it shouldnt have caused any damage.

Moreover, this engine will only be in this car for this year while I complete what I hope to be a 500hp 347 from scratch. Then I'll put this motor in a go kart and terrorize the neighborhood

 

Just about, yeah. Just wait until it cools after that first run and check the level. Since the coolant is supposed to be 1" below the filler neck, it usually works out that you are good to go.

That should be no problem. I scan new engines being run-in with an IR pyrometer, and head temps up to 230° are pretty normal. They usually get 15-20° cooler by the end of 30 mins.

 

Yeah, maybe not in a stock rad for a 65-66. But I have done a new engine and had 1/2 gal trapped in there after it starts and warms. But I usually have a radiator in there that is frame rail to frame rail. Pinhole works for me on the smaller stuff once the t stat opens. The tilt seems to make the process much easier and faster.