Replacing corroded or leaking freeze plugs is a necessary repair to maintain the integrity of the engine’s cooling system. Core plug , freeze plug , also called a Welsh plug. The purpose of Freeze plugs: they are installed in the engine block and cylinder head to seal off the passages left over from the casting process (where sand cores were used to create the internal cooling passages). Their most critical function, however, is to act as a “safety valve.” If the engine’s coolant freezes (due to