Choosing piston

MOPAROFFICIAL said
"Exact" is like within .006 lol. Jk
Compression height/pin height.
And that's based on a rod length being blue printed, a stroke blue printed, and an OEM deck height being blueprinted.... aka perfect.

There is always a hair variable.
So the rod is .002 shorter.... and the deck is taller by .008..max and .003 at the small end.. but the machinist just squared it and took .007 at one end and .002 at the other.. and the mains were line honed... etc... it's a stacking of clearances

The reason you go to a competent shop is to eliminate those stacked clearances. You rebuild the rods (or redo those import rods) so each one is spot on. Both decks get squared and cut the same. You check the pistons to be sure the pit height is the same in every one. You must check valve run-out on every valve not just assume they are good to go in those shiny new heads or the "just rebuilt" factory heads from eBay.