Anytime you change one componentlike the rods or pistons, even the wrist pins in the rotating assembly you should have it check for balance The factory pistons are heavy, so are the six pak rods, but in a different place You have two different weights that are looked at and measured when balancing rotating masses, the recpricating(SP) and the actual weights of each component are matched so they weigh the same as the lightest individual components in the crank assembly, all rods weigh the same on both ends, all pistons, wrist pins, rings, bearings and so on weigh the same