Pontiac used different thickness on their head gaskets. I see 2 part numbers used, 9792995 - .028"-.033", and 9790910 - no thickness listed. "995" is used by 1968 F-8/T-8 400 4 Bbl, 1968 P-8 HO, 1969-70 All 400 4 Bbl except XV/XZ engines, 1969 P-8 428 & HO.
I also read that the chambers will measure a few CC's larger due to the fact that the leaded gas would carbon up both the combustion chambers and tops of the pistons reducing CC's, so Pontiac took this into account.
And, advertised compression ratio's were marketing ploy's so as to measure up to competitors in an effort to get sales - just as cubic inch sizes were comparable.
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