What I use is Arm & Hammer Washing Soda (NOT BAKING SODA). You can purchase it in the laundry isle of the supermarket. Drain the entire system by pulling off the lower radiator hose at the radiator. Be careful not to damage the radiator hose nipple, by using a pointed tool designed to loosen the hose without damage to the radiator or the hose. Use of this tool is important, so you don't break the soldered joint. Next, remove the thermostat, and replace the thermostat housing. Then remove, swap, and reinstall the heater hoses on the engine side of the heater hoses. This is important, because you can easily damage the heater core hose nipples, if the rubber is stuck to them. If you need instructions for removing them without breaking them off, let me know. Reinstall the lower heater hose, and clamp lightly. Now you are ready to start cleaning. Dissolve 1 cup of washing soda in a couple quarts of water, and pour it into the radiator. Fill the engine and radiator with clean water, and make sure that the heater valve is closed. Once the engine and radiator are full, start the engine, and at a fast idle, allow the cleaner to work for about 15 - 30 minutes, or until you see it turning a bright orange color. Shut engine off, and carefully pull the lower radiator hose off, allowing the cleaning solution to exit quickly. Allow the engine to cool until you can put your hand on the engine without it being hot. Usually this takes about 1/2 to 1 hour. Reinstall the lower radiator hose, and repeat. Do this until the water is clean and white looking. Then, open the heater valve, and allow the solution to clean the heater core. The reason for reversing the hose connections, at the engine, is so the heater core is flushed backwards, so it doesn't clog. Allow the solution to flow through the heater core for about a half hour. Then pull the lower heater hose one more time. Remove the two heater hoses from the engine, and point one toward the ground, and flush the heater core with fresh water from a hose. Do it in both directions, to make sure that everything in the heater core is removed. Put fresh water into the engine, and run till it is warm to hot. Drain from the lower radiator hose once more. When the engine is cold, I would replace all heater hoses, radiator hoses, and thermostat with new parts. Add pre mixed anti freeze, and you should be good to go for another 3 - 5 years. I prefer the premixed anti freeze, because it is made with mineral free water, and it is also PH balanced. If you are not sure of the engine, it might be wise to check the water pump also. Many times, once you clean everything, they start to leak. The crud in the system acts like a stop leak. The Washing Soda will not cause any harm by itself, but if the engine has years of neglect, you might find that the freeze plugs will also start to leak. If that happens, you might as well replace all of them, because once one goes, the others are not far behind. They are a pain to replace, especially those at the rear of the engine and the rear of the heads. Sometimes, I had to partially remove the engine to get to them all. I have used this method of engine block and radiator cleaning for over 50 years, and have never experienced any problems as a result of the cleaning. I have experienced failures that were a result of neglect. Better to have the cooling system fail now, then later on when you are on the road. The washing soda is a cleaner, that will not attack the metal parts of the engine, only remove rust and scale.