scubaozy

BobH, I think you are quite close. Yesterday in the club I shared the images with our clubs experts. One of them is main importeur of various engines from far east. He told me exactly what you said. Overstress because of either advanced ignition or too much fuel in the chamber or combination of both. He has in the past certain engine models being returned with a blown cylinder head. The engine was in fact quite nice but lightly build. It would crack at the weakest spot. They have retarded the ignition for all the remaining stock and they did not hear any problems after that.
Retarding the ignition or higher octane fuel might certainly improve the situation. There will be some power loss in retarding but quite frankly, I already had it difficult to reduce the max rpm with h needle. Everytime I reduced top end a little, it would start with misfiring. So, retarding might solve that also.