just got my new set of pistons and wrist pins (damn press fit) and the rod and piston weigh about 2 grams more than the old ones. Would it be necessary to have my crank rebalanced or should i just mill the 2 grams off the piston? or not even bother?

 

Plenty close. No need for a rebalance.

 

7 grams adds 28 lbs of external force at 4000 RPM. 63 lbs at 6000 RPM's. I would mill them if you have them out already. It would be less ware on the bearings at least.

 

There are multiple weights to a piston assembly.

All the pistons with pins should weigh the same. Not all pins can weigh the same unless they're all from the same batch.

The rods are all matched the same weighing the big end and the small end separately. All the weights are brought down to the lightest one on both ends. Different rods from different batches can have a big enough difference in weight that they can't be matched. Same with wrist pins.

Once all those weights are matched, the weights from rings and bearings are added to get a bob weight.

2 grams may not sound like much and will probably survive with no problems in a street engine.

When I had my rotating assembly balanced, I had the crank spun up with and without the flexplate. The flexplate was out 10 grams. The assembly was balanced with the flexplate to take out that 10 gram imbalance. I also spin my engine to 7000 rpm.

The engine guy said that you can normally tell how bad an engine is out of balance by watching the antennae vibrate at an idle.