This is actually a pretty common high hour failure, in my experience usually on exhaust valves.On alot of Cat's valves the stem and head are actually made of two different pieces of metal that are welded together, and they usually break near the weld. Try sticking a magnet to a valve; it will stick to the stem, but not the head. Cat says that excessive valve clearance can cause this failure, because at high rpm the slack causes the momentum of the rocker arms to "throw" the valves open and slam shut, causing metal fatigue and breakage. Of course it takes a long time for this to happen, IE high hour failure. If an engine rebuilder puts old valves in a rebuilt engine, the failure could be percieved as a low hour failure, but its not. Like RKO says, new valves should always be put in a rebuilt diesel. Corrosive pitting in the fillet area is also a common cause for this failure, but it is exclusive to exhaust valves. If the valves are non Cat made, they should be removed from the engine, and melted down to make tourist souvenirs, LOL, and real Cat valves installed.
