Obviously the plastic based head isn't capable of enduring the high heat within the combustion chamber, hence the failure of the threads.
I recall days at Fox Mfg that in lower light conditions, within the exhaust port could be seen white hot gasses being exhausted. Also, the heat created during running on a well fitted engine will be localized from above the exhaust port: Fins/head. During running, the lower portions of the case will actually be cool to the touch. (I've touched bearing areas/etc countless times feeling for vibrations, abnormal heat, etc.) It's only after shut down that the heat QUICKLY spreads from the head/upper cylinder to the rest of the case.
Glow engines are fascinating pieces of mechanization. I never cease to be amazed at them.