Maybe once or twice a year and or when I mount it on a new plane . Or when I run a pump, Cline, or differant muffler. etc. Or in this case I swapped out an engine and was getting the 160 cleaned up for storage.
My guess is the head bolts got a tad loose and it stuck to the sleeve. But I've never seen one do this unless they were way over heated. I probably need a new gasket is all.

Thanks for the replies all.