The poor heat treatment on early failures is a very common theory, and one I suspect has merit in many early failures. However, it's been disproven on other failures via hardness testing and wear pattern analysis. Interestingly, wear pattern analysis also indicated sufficient oiling, another common theory, but perhaps *improper* oil and additive packages. There's merit to the posts here about various oil and additive packages addressing the "tick" as well as longevity.
Another theory I've heard thrown around is improper break in procedure/insufficient assembly lubricant during assembly of the motor. I don't know enough to evaluate that claim, but it's been tossed around as another explanation for early cam 'death' as well.
This is a free example online of such testing results:
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