Interesting topic.  It begs the question (I've always wanted to say that), should all rebuilt engines be rebalanced, if they use new pistons, or rods?

Some of the other guys may have a more experienced opinion. I always have the rotating assembly balanced in the motors I build L6, V8 whatever. For the added costs compared to the total rebuild cost I do it. I don't have to pay someone for assembly so I come out ahead overall. A balanced motor should have less vibration and theortically rev smoother and potentially higher depending on the valvetrain. The last time I paid for balancing it was $300 or so...

C

I'm getting ready to test this subject. I've swapped out the crank but I am using all the same rods and pistons. I don't plan on having the complete rotating assembly balanced.

I don't think you'll notice as large a difference in the L6 as you might in a V8, simply because you have no opposing masses.