Pistons in hand when boring?

At the factory, they bore the blocks to the block specs, and the pistons are made to the piston specs. If they are both made properly, they will all work together and not need to be "special fit"....

But for one engine at a time, you are better having the machinist make it to the pistons once you decide what bore diameter you are going with and then order pistons for that. Then he can make sure the pistons will fit the cylinders properly. You can control your clearances better that way....