I have seen and heard of as much as 18 grams differences in stock pistons and rods, most good balance shops shoot for one gram variences on rods(with bearings and oil), pistons (with wrist pins and rings) on there balance jobs. Some achieve that and some don't I have a Haus triple beam balance scale so I do check the parts now and then just to keep the shops on thier toes BTW, to do balance checking properly you need a shadow graph and a crankshaft balancing machine, which I don't have