!984 GMC Heads

Is it true that the older the head, the thicker it is. I'm on propane, always cracking my heads. Other trucks in the fleet were also on propane, different years, different makes, different models, but never
seemed to have a problem with the heads. I've always been a fanatic about maintaining my vehicle, changing my oil every 4000 miles, tune ups every 6 months etc. Finally a friend told me about his theory about the older heads might be better, so I threw on a set of heads from a 1980, and sure enough, "no cracked head". My rings finally went after 200,000 kilometres, but since I got my full core exchange back, I would assume that my 1980 heads still weren't cracked. But guess what, my new motor, after about 70,000 km's now have a cracked head. Is this just a coincidence, or do you think my friend has a point. Any input would be appreciated....thanx in advance...
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Nothing to do with propane. You have a cooling problem.

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