All flywheels are balanced. That is what the drill holes are for. If they weren't balanced, the engine would literally shake apart in no time.
In general, most flywheels are "neutrally balanced" meaning they do not play a role in crank balance. The crank and flywheel are balanced separately.
For some reason only god knows... GM used the flywheel to balance the crank on some V6 models. In this case the crank and flywheel are balanced as an assembly if you want the best balance posible. This has always been a nasty way to balance a crank.
Note that if the crank balance wheight is milled off on the flywheel, the flywheel needs to be rebalanced. Simply milling it even isn't accurate enough. Even small amounts of imbalance can shorten engine life considerably.
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