I have a balanced assembly, crank,rods,pistons. I am wanting to switchout the pistons to gain more compression. The pistons balanced to the kit weigh 658grams, the pistons I want to use weigh 640 grams. Do I need to rebalance the setup or will it be ok?

Absolutely a-ok. I wouldn't worry at all. J.Rob

That is known as a crank OVERbalance .... this is cool.

there is 28.? some grams to a ounce, so the new pistons would be around 2/3 of a ounce lighter than the old ones. BTW, does the piston wieghts you furnished include the wrist pins on both or not? Are the rings the same sizes on both sets of pistons, IE 1/16, 1/16 , 3/16 or are they different sizes like 5/64 on one set and 1/16 on the others?

The piston weight difference is about the magnitude of doing a 50% balance compared to a 51% balance (or 49%). So if you have a reason to need other than the normal 50% balance, that is in the ball park.

But if you want a balanced assembly, and the normal 50% is right for the application, then it needs to be rebalanced to match the new pistons. I would rebalance the crank.

They are same ring size, weight is without the pins. One set is Ross the other is Diamond.