The JSM pistons for stock rods are lighter than the original stock pistons. When you lighten the pistons (reciprocating weight) and do nothing else you reduce the vertical shaking of the motor but the horizontal shaking stays the same. This holds true until your balance factor reaches 100% but you are never going to be able to lighten the pistons enough to reach a 100% balance factor. So your bike and motor will feel smoother and stress on the crank and cases will be reduced. Your balance factor will increase and if you want you can rebalance the crank back to stock balance factor specs to accomodate the lighter pistons. This is all very fine. But either way the lighter pistons will give you a smoother ride whether or not you rebalance the crank.
Examples:
A bare stock medium 850 piston weighs approx 317 grams
A JS 850 medium compression piston for stock rods weighs approx 255 grams.
An 850 with the lighter JS pistons for stock rods and no other changes will increase the stock balance factor of 52% to approx 58%
