topped off using six gallons of distilled water. This way you get the radiator flushed aswell. Once the engine warms up enough for the thermastat to open it swallows that six gallons of water very quickly...probably in about 5-10 minutes total. The plug is just to keep the mess out of the engine bay but with the hose clocked upwards like that and being the highest point you may not even need the plug.