looking at threads on replacing 1 or 2 pistons. pistons get heavyer as you go up in over sizes, and makes eg, a polaris piston of .50 over size is heavyer than a wsm .50 piston by aprox 20 grams. this leads to a lot of differant weights on the crank . so check the weight of your pistons and try to match as close as possible to reduce strees of uneveness.