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Col is correct, on the engine the in is at the water pump,this goes to out on heater(lower connection).... out at the intake manifold, this goes to in on the heater(upper connection
Jim
Only issue is getting all the air out of the heater core. Filling from the bottom will do that more easily, but you could probably do it filling from the top as well.
it will work eithier way, but according to a freind who was in the radiator bussiness for 45 years, filling from the bottom removes air pockets and allows for better heating, also was said in the instructions for a aftermarket heater/air unit I installed, and Ford shop manuels also show it connected that way