Balancing an engine

Moper described it best, that weight/balance tolerances are pretty wide in OEM applications, but were "acceptable", especially for everyday street use and lower rpm's. But they are likely better now. Just because you can't control every tiny detail, like crank twist, doesn't mean you should just let it all go. You can drive around with unbalanced tires. Some may be off enough to feel the vibration and some may not be enough to feel it. But it's not as good as it could or should be. Imagine spinning a top with a little extra weight on one side! Then remember that engines can go 6,000, 7,000 or more rpm which magnifies any imbalance considerably! Matching piston and rod weights as well as balancing the crank counterweights to them can only help HP and reliability. Any serious race engine has it done and it's a good idea for any performance engine, especially when parts start getting changed. If you can find a dependable shop and can afford it, do it.