TRW listed the LT1 .100 dome piston as 11:1 w/64cc. Sealed Power now lists it as 10.63:1 w/64cc. Is the piston the same with a closer comp listing or has the size of the piston above the top ring changed?

Are you sure you're not looking at a new generation LT1 part number??

I'm looking at my old TRW book and the Sealed Power listing in Summit and that may be the problem.

They changed the way they figure quench. They now use the factory nominal deck height (.025" for a Chevy) and a .038" thick gasket. I just read this in their catalog a few days ago. I'm not sure how they did it before but it was different.

I have the older TRW and my dad has the newer Speed Pro not Sealed Power. I had forgoten that the Speed pro is smaller in width above the top ring, not even 4" on a 4.03 piston.