Ok, calling all big block wizards. I am trying to figure what my compression height needs to be for pistons. Actually I knew, based on 9.8 deck height, but I ran into something a little confusing. Deck height for non-truck MkIV 454 is 9.8. Then I start running into (true deck height 9.78 ) always in parenthesis. So what gives? Which is true? I always thought there was one deck height. Period.
It's been a couple years since my machinist zero decked the block. Or did he? 9.8 or (9.78) ? He doesn't remember.
I'll dig the block out of hibernation and take it to him this week and we'll re-measure.
I'd sure like to hear explanations about that variable deck height.

Found the answer. Nominal deck height for MkIv is 9.8
True or zero deck height is 9.78