I've often seen the following advice on this board in regards to flushing the cooling system: drain and refill the radiator with distilled water until it runs clear, and then fill up the radiator with straight antifreeze to achieve a 50/50 mix; fill up the overflow tank with a 50/50 mix (did I get that right, or have I misread/misunderstood something?) Here's an argument against the final refill portion of this method that's been presented to me a little while ago: since the contact surface area inside the engine/radiator for the 2 fluids is so small, the fluids will not mix effectively. Furthermore, any and all antifreeze/water mixtures should be produced outside of the engine for the same reason. Personally, I don't see how it would be possible for the two fluids NOT to mix inside the cooling system. After all, if that was the case, then the first part of this procedure would not work, since the water would not mix with the old coolant. What do you think?