I have a 85 yahama it200
exhaust did leak at he head
no air filter (was testing engine due to not knowing filter was junk)
brand new carb factory not china

exhaust side of piston melted and scored horrible 1 hour of run time after new piston caused engine to stall - bad enough to where I'm getting it bored and new piston again the piston rod was also bribed so I'd didnoverheat for sure
smoked like teenagers trying cigarettes for the first time - was plenty of oil.

i am getting new exhaust gasket
new lower end seal
Checking carb settings

what else could cause this so I'm not doing it a 3rd time???


This his was a basket case - engine was tore apart when I got it - old piston was good but could not get factory stock rings. Backordered but it was not rebuilt because of this issue

 

Piston scoring on exhaust side is usually because of heat or air leaks plus at times jetting. If the jetting is good then the new gaskets and crank seals should solve the air leaks plus check the intake manifold also for any splits or cracking.