The disc was the tip of the valve. Fortunately the piece broke off and the keeper clips stayed on. I have no idea what might have caused this. Maybe it was something pre existing from the previous owner of the heads? Too much preload? I don't wan't to do anything until I have an idea what caused it. I have a fresh set of smog J heads with new valves and springs that I am tempted to swap out for the aluminum heads. This is frustrating!