Exactly, It isn't heat from combustion, it is heat from friction due to lack of lubrication from a lean air to fuel mixture. The exhaust side of the piston usually sustains damage first if you displace fuel (oil) with air. The exhaust side is easy to see. You can also check the intake side (which I would do if the exhaust looks good), then check the stator side and then the clutch side as I said previously. If you find nothing after all this, you may be faced with spitting the cases to find the issue.
SP