valvetrain, and we pulled it apart after seeing what happened to the Peacemaker. It also showed the same signs of severe valve-face wear, but we caught it early enough that there weren't any failures. All of the intake valves in Russell's engine were reaplaced with inconel counterparts, and haven't had any problems since then that I am aware of.
I dont have an answer to your question though as to why no one else has had this problem either. Then again, not many others have made the same amount of power as The Peacemaker and Russell's widebody have made.