I did my 912 some 15 years ago. The heads had cracks in them and at that time Harry Pellow suggested Competition Engineering, Ray Litz, could fix them. He did a marvelous job. So you might want to contact Walt, now Competition Engineering,
http://www.competitioneng.com/Welcome.htm.

"It's not the overrev on the up-shift that kills a motor, it's the over-rev on a downshift that will toast it. And there isn't a rotor-button around that will protect against that."

John: i think you've missed my point. i'm pretty sure it was a downshift over-rev, but it didn't occur during htis event. i think i must have done it earlier in the year and the valve just took some time to fail