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Discussion Starter #22
I don't think it was caused by coolant. There's no sign of the steam-cleaning effect in the cylinder, and the head gasket looked unbreached. I don't know if it's visible in the photo, but the deformation of the valve looks like it was caused by heat projecting upward from the cylinder. If the carbon held the valve partially open briefly, there may have been enough compression for it to fire, eroding the valve further and further until compression was no longer possible.
 

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Discussion Starter #23
Quick update. Got new exhaust valves and valve stem seals. Stripped down the head, cleaned everything, inspected - everything looks good, intake valves quite clean, no pitting in the seats, guides nice and tight, no evidence of corrosion anywhere. The old valve stem seals were hard as rocks, but that's to be expected. Lapped in all the valves, got the intake side installed with new seals this afternoon. Hopefully get the exhausts in tomorrow. Then it's just rolling in the new timing chain and bolting it back together.
 

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Just reporting in. If the motor closed on something there be depressions on the seat. Only affected one intake valve, no steam clean from coolant (although it looks more like a lean condition issue). C900 injectors don't have a well defined spray pattern like T5 and up motors so a clogged injector doesn't really make sense. I suppose abundant pre-detonation due to a lean condition could pick on that valve specifically, fracture the valve face and cause this? I'm grasping at straws, not even on a cylinder farthest away from fuel pressure. I have no idea what happened here, carry on...
 

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Discussion Starter #25
There was a lot of carbon built up around the point of failure on the valve in question. Exactly why, I don't know, but it wasn't present on any other valve in the same concentration.
 
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