Rustler wrote:
>
> But if you run the
> car for half an hour then drain it out again, according to directions, is it
> dangerous to then put in the new antifreeze and cold water?

Normally, the engine will have a thermostat that keeps the coolant temperature
above a certain point (usually 180 or 190 degrees). It should be obvious that
the coolant isn't going to be much higher than 200 degrees, since even pure
antifreeze boils at about 240. Shut down and drain the system, and this will be
roughly the temperature of your block. A temperature difference of 150 or 200
degrees isn't going to stress the metal that much.

> Or, is it a problem
> only when the engine has been overheated, for example from a lack of coolant.

Bingo. Now you're talking differences as high as 1200 degrees. Especially with
cast iron, that's enough to cause fractures.

George Patterson