You have pistons and rods that are fine and individually weight measured to each other statically. You have a flex plate and a harmonic balancer that are probably fine as individual parts as they are usually spun (dynamically) balanced. But when you look at the single biggest/heaviest part - and even if it IS balanced as an original part, is NOT balanced to the new weight pistons - and they are a different weight then the originals as are the rods. Here's a very basic write up - if you need more, do a Google search as there is lots of good info on line: Understanding Crankshaft Balancing: Engine Builder