Took it apart.
First look as soon as I removed the head:
The complete ring where just the carbon is knocked off shows that the valve was riding the piston for a little while. This must've been during that time when I was cruising, had just noticed it and was trying to determine if the odd vibes I was feeling were from the driveline or a loose wheel. It was the steady rhythmic pulse of a dead cylinder. Had I known, I would've shut it down that instant and all I would've needed was a new valve spring which would've been a 3 hour fix on the side of the road to get me back home with no damage at all. But, at some point, the valve and piston had a disagreement and while the piston was trying to buck the valve off it's head, it bítch slapped it sideways, leading to the hoof marks in the piston:
Pulled a valve from a junk 706 head for comparison:
The head looks fine. It looks like it did little more than just knocking the carbon off. I don't know how it survived, but the guide looks and feels fine, too:
I buffed the carbon off the piston and smoothed the hoof marks to eliminate sharp edges that would lead to hot spots. The light reflecting makes them look more severe than they really are:
I pulled the lifters from that cylinder and they look fine. Plan of action is to clean up and install that salvaged valve, replace all 16 springs, reassemble and send it.
