I am going to buy a 408 stroker kit either this week and or the next. They will need balancing, but it says itisbalanced externaly. Does anyone one know what this means?

 

External balance means it has to be balanced with the harmonic balancer and flywheel/ flexplate installed on the crank.

 

Yeah. Internal/external balance deals with imbalance factors. The balancer/flywheel in external balance has weight offset in it, that combined with offset weight in the crank will cancel out vibration. Balance factors are 28/28, 34/50 etc. Like the later 5.0's are a 34oz offset weight in the balancer and a 50oz offset weight in the flywheel. The weight doesn't actually weigh that much, it's a measurement of the amount of offset force generated at a certain rpm. Because the weight to help balance it is external of the engine, they're called externally balanced. It's by far the most common.

Internally balanced are also known as 0 balance because the balancing is done entirely in the crank. They use a 0/0 imbalance factor, where there is no offset in the harmonic balancer or flywheel. They are a superior setup in terms of vibration control and performance, but are much more expensive, and can be VERY difficult to balance. 0 balance factor setups usually require welding a heavy metal, such as tungsten, into the crank. The material itself is expensive, it's time consuming, and they are more difficult to balance than an externally balanced setup. You usually only see them on race cars and very high performance street setups where more thought, and money, was paid.