The heater core is the 2 rubber hoses going into the car. I believe he used bulk 5/8" heater hose to connect to the heater core. Line in for water and chemical and a return line to a bucket. Regarding CLR it says not to use on aluminum. Plenty of people do use it on heater cores. I would try not using too much. Flush out really well. I have never done| this. Just sharing what I know .