We installed a reman cylinder head, new cam, some new rocker arms and one new injector in a Series 60. The truck is a 98 Kenworth. After repair the owner operator claimed that the engine never ran this good and that he had improved fuel economy along with way more power. At approx 5000 miles after repair an intake valve dropped on #5 destroying the head, piston and liner. We sent the head back to the original machine shop and they are claiming that all the valves are bent. We checked valve lash before disassembly and all valves were right on. I am searching for a possible cause other than an over-speed.