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yes, you right about the head gasket.should be 0,039"

I used the shims for compensate for decking and millig the heads.What can I do? shorter pushrods or adj rockers?
peter



The correct way to make up for the head and deck milling is to get the correct length pushrods , in this case shorter .

But this has little to do with your detonation issue, you'll have to tune it out.

I wonder if your plug choice has an effect since it's one step hotter than stock ?