[mstngsule68]

Originally Posted by Jfsram

This really depends on how much you want to flush it.

If you just drain it like the first reply said. You will still have lots of accumulation in the engine block and heater.

I prefer the disconnect the hose idea, especially with the thermostat removed. You should replace it when you swap out the radiator anyways.
For a real thorough flush remove both heater hoses and blast water through it both directions. Then through the block too. Catch the rinse water and keep repeating until it flushes clean. A neglected engine will need much rinsing.

Now try to pour distilled water in the engine so the tap water is displaced. Install the new radiator and make sure to get in at least half of the cooling system's capacity filled with antifreeze. Most cars are 1 to 2 gallons. Then top off with distilled. This should give you a 50/50 mix.

you can also pick up the little tester that tells you how well you actually mixed the coolant.