Got new pistons for my small block. They weigh 16 grams each more than the old ones. Do I need to get the motor rebalanced?..8500 RPM

Can you grind each piston down enough to lose those 16 grams? If so, do it
if not, take it to a good balance shop

Deepen the valve reliefs, create flame notches, shorten the skirts under the pins (non-thrust axis), but aluminum is 1/3 the weight of steel.
In some cases, the piston pin can be taper-bored from both ends, go slow, keep it wet & cool.

I would. under balanced is not the best way to be.

16 grams will be a tremendous task, especially on all 8. 16g's is a lot of aluminum.

I would first consider a different wrist pin. If it is chevy size, they make a lot of options.

Is there a lighter wrist pin available?

On a 5000 rpm engine 16 grams matter. On a 8500 rpm engine 1 gram matters.

Yes you need to rebalance.As previously mention taking 16 grams off the pistons might be tough to do. Changing the wrist pins is more probable, but you should still rebalance the engine.

Tom