The heads are aluminum. The heating up and cooling down causes them to warp. It can no more be prevented than you can prevent the piston rings from wearing out. There are things that help, like keeping the carb tuned so it isn't running lean (hot) and replacing the head gasket every 5 years or so..... but it can't be totally prevented.
By the way, they can be planed..... replacing them because they are warped is completely unnecessary.
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