hey guys am i suposed to fill my rebuild 350 with antifreeze threw the thermostat before i start it or do i just fill thje radiator??

 

Fill both. Then run it with the radiator cap off to top it off.

 

I just fill the radiator, start the engine with the cap off, wait for the stat to open then add coolant as needed.

 

Well, if the engine is totally dry of antifreeze, and the radiator is full, there might be a chance that the pump could just spin or cavitate, and not really pump anything. I would recommend doing what rezinn suggested.
Get it as full as possible before you start it, then top it up with the engine running.

 

there's a chance a big rock from space will fall on you too and none of this would matter. i've filled well over a hundred new or rebuild engines by pouring coolent down the radiator without the first problem. i'd leave the t stat alone.

 

LOL.

 

Old-timers used to force the thermostat open by hand before they put it in, and prop it open with an aspirin tablet to let the air bleed out... but like ede said, it's so trivial it doesn't really matter. Just fill it through the radiator and let the engine circulate the coolant, then come back and check it when it has cooled down after running and top it off.

 

Fill with H2O first then break in the engine. If all is good then add 1/2 anti-freeze - 1/2 water.

Here is the reason. If there was an external coolant leak, you loose anti-freeze. Water is a lot cheaper.

If you have an internal coolant leak, you won't know it till it overheats. Ethylene glycol is VERY nasty to bearing. Water is not nearly as bad.