Yes, on the last test run before heading for Spa trip next weekend, cylinder 4 intake valve broke. It shouldn't, no high rpm, and fully warmed. As pictures show, broken rocker, 2 valves, piston with deep groove. Looking at the valve stem on the faulty valve, it looks to me like a mismatch between retainer and locks. I'll bring the head to a local machinist Monday, I'll want his view as well. When playing with a retainer and locks with no valve stem inside, the locks touch each other at the bottom, but there's 0.6mm gap at the top. Looks mismatched to me. The head needs a new seat and a valve guide, hopefully that's all.
The engine is a MME Racing 408 dynoed at 593HP. I called MME Friday, they were interested in helping solving the issue, but of course Mark wanted to see the pictures. I sent pictures, but I didn't hear back from them Friday, I'll call them Monday. The engine is 4 years old, but only has 4000 miles on it.
I'll keep you posted.
Sad pictures below