Looks like my spark plug. I still don't know what caused my problem. I was driving home from the gas station about a mile away, driving like a granny. Not getting into boost or anything. All of a sudden CLANK CLANK. Coasted into a parking lot. Removed the head and found a valve laying in there. The stock valves are a 2 piece design, and where mine was broken seemed like the spot where the 2 pieces would have fuzed together. So it seemed like just a random valve failure in my case, and that's the best I could come up with. I feel your pain, sucks man.
I fixed it the cheap route. Replaced the head with a 2G head. Replaced the Piston/Rod in that one cylinder. New rod bearings throughout. Bada bing.
http://mitsustyle.com/forums/showthr...t=wtf+clanking