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That i don't know,but by the looks of the valve it probably happened at idle or just off idle,otherwise i think there'd be more damageWas the MDS being used? A new head gasket and some heavier oil and she'll last another 100k.

The odd one does break.They're a mass produced product,that is subject to wear.Yeah I had a valve spring break too. I assumed it was a rare failure, is that not so? My intake valve didn't look nearly that bad and the piston was OK.
I did all the repair work myself but the machine shop bill was substantial -- 16 valve springs, 8 valves, plus machining. And cam and lifters and all the other stuff
I did while I was in there. If it now lasts a long time it was worth it... but it was a big job for sure.