Strangly, or not both pistons are free to move in the cylinders and don't appear damaged at all. I'm 90% certain this was heat related rather than a mechanical failure. Now whether it was due to a lean mix or leaking coolant (it had a pinhole leak but was topped prior to the race and had completed two prior 8 lap races) or failed water pump (using a non gycol coolant for racing that was of cooking oil consistancy!?).
I had an external temp gauge on the top cylinder since the dash one was unhooked. Pitty I didn't have a baseline having never run it before, after my first few races the outside of the top barrel was 66 celcius or so - pretty hot. I know my RG150 runs 40-55c usually (i'd seen it hit 70+ before) so I wasn't too concerned.
I'll add pictures, hopefully you guys can give me some advice. One cyl wasn't firing all the time at idle, but kicked in no worries with a few revs and on the track it was pulling plenty well.
Running 33:1 premix fully syth castrol TTS with 98RON gas, standard jetting airbox minus filter but not drilled or anything. No sign of piston contact (scratch on the carbon of the top cyl pic is just from a head stud lightly touching it when removing)
Bottom head
Top head
Bottom piston
Top piston
Top cylinder plug
Bottom cylinder plug
So the top looks to me like its been running and the bottom very little, although completely differently from the sparkplug electrode color? wonder if one of the carbs got blocked. Top cylinder plug ok or a bit lean?
Is 66c too hot etc,
What say you wizards
The ring on the bottom piston is from before head machining, it wasn't contacting - and you cant see it on the top piston from the carbon