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As a very general rule, the newer heads with the "extra" 2 head bolts in the timing cover all had hardened valve seats. (Easily identified by only 2 bolts holding the thermostat housing to the head.) This is not to say they had a seat of some hard material machined in, which is what you have to do to the older heads, but instead the material of the head itself was hardened by a process known as induction hardening. While this did have the desired effect of greatly reducing the ammount of valve and head wear with unleaded gas, it unfortunatly also made the head more brittle, and more likely to crack and fail. Usually a single trip into the temperature ranges >220 will destroy these heads.
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