Pulled the head tonight and #1 exhaust valve snapped the head off, flipped over in the cylinder and punched a big hole in the head, plus cracked the block. I think the block can be sleeved so if I can find another piston we may go that route as a spare motor (only has about 50 runs on it). The Dick Landy head is toast. The valves were Ferreira stainless versions; not cheap and the guides were bronze-inserts. Everything was checked by my machine shop earlier this year, so the only thing I can think of is component failure or the valve stuck in the guide. We had Teflon seals on the valves and they may limit the amount of oil to the exhaust (just a guess on my part); they may work too well. Piston to valve clearance was more than adequate and there is no evidence of other valves contacting the piston (if anything, the intakes would hit before the exhaust). Major bummer It's all Mr. Yuck's fault. His engine is contagious.