Originally Posted by Zuldarin
I know this is a long shot but are you sure its the piston hitting the top? The rings on these engines do have an upside and a downside. If you install the rings (any 1) up side down the piston will not go all the way to the bottom of the tapered barrel. You should have noticed this when installing/gaping the rings but just a thought.
DId you gap the rings? I just bought 6 superior cly kits and on each one, I had to shave a good 10-20 thou off the rings to get the minimum gap up in the choke area. If I didn't do this, I would have had the symptoms you describe.
That said, if the cyl mounting decks are milled, you definately don't want high compression pistons.