How far can you deviate on piston weight before needing to rebalance?

[travis]

For a purely street/utility use 351w that was balanced with a set of 790 gram pistons/pins, would you rebalance after to going to a 21 gram lighter piston and pin? This thing will rarely, if ever, see 5000 rpms. I am concerned about long term durability and having a smooth running engine

[rustbucket79]

You'll never notice the difference but if you can afford, especially if you need to replace a flex plate or resurface your flywheel or replace your harmonic you may as well balance. Much better than wishing after the fact.

[Ericnova]

Being lighter like that wouldn't even make me blink. Run it.

[MadBill]

Lots of engines are deliberately 'overbalanced' by more than that.