On really badly clogged radiators like the concrete mixers I used to maintain that got full of cement dust, I used technical grade Hydrochloric acid (Called "Muratic" acid at home improvement stores and swimming pool supply places). It's extremely aggressive, and can open up pinholes in radiators that aren't in the best condition. Pour In 8 ounces or so, fill with water, and run the engine at a fast idle for 30 minutes. Drain, flush with clear water, and add a box of baking soda to the next water fill. Run the soda solution for at least 10 minutes, drain and flush, then fill with a 50/50 antifreeze/water mix.
