[Sundancer268]

If it is an ATWOOD, do not replace the plastic plug with a metal one unless you can find an Aluminum plug. Atwood uses plastic to prevent Galvanic Corrosion at the plug thread tank thread interface and the plugs do have to be replaced every so many years as like most plastics they do break and harden with age. How soon the Galvanic Corrosion will happen will depend on the quality of the water you are running through the heater. Look in your manuals and brochures that came with your unit and it should say what brand came with your unit.