Why unlikely ?

Because I would have expected the inital complaint to be boiling over or spewing antifreeze. Also, as far as things go, these things usually will burn a gasket between cylinders and/or melt valves before they crack a head. Overheating will usually induce the crack. The lack of valve adjustments usually will lead to the burning of a valve.

That, is why. It is not impossible, just a little farther down the list of possible things.

It is all academic anyway, the head needs to come off one way or another.

Michael