[ben333]

I've heard many things, some people just use water, others suggest toilet bowl cleaner. If it's a regular rad, not clogged too bad then I'd recommend starting with vinegar. Pour some in, hold your fingers over the rad's fittings and slosh the vinegar around inside. Don't let it sit in there too long, it's good for getting rid of corrosion but can also eat the metals. After the vinegar, I'd probably make it a priority to thoroughly rinse the rad. After the vinegar, I myself add a little dish soap and let the tap water run as hot as it gets. I fill the rad up, leaving a little air left inside to increase the effect of shaking the soapy hot solution. After that I rinse a few times with hot water, washing out all metal bits and flux from the rad itself or other parts in your loop. Other crap should come out too. Rinse till the water coming out has no crap or bubbles in it, and you should have a pretty clean rad. If you used hot enough water, it should help the inside dry out faster too, still try to shake out all the water you can when done though. Some people rinse with distilled water, I'm not that paranoid though, knowing soon that the radiator will have clean water flowing through treated with a bit of antifreeze (in my case anyway) so a little bit of tap water left shouldn't matter too much. (Don't leave any cleaner in though!)