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There isn't one single car engine in the world that isn't balanced from the factory as part of the normal manufacturing process.. Its either balanced or manufactured and held to a certain weight that is controlled in the process.
Depends on the method in which they manufacture the part. For instance lets refer to a crank shaft. Typically the forged (heated, pressed to shape, and machine) parts require balancing where as billet (machined from full size blank) parts require very little. It is still equally important for ensuring a long life of the bearings supporting the rotating objects. Bearings last the longest when subjected to equal loading (and obviously lubrication).
Beyond that there is a difference between a balanced part and a balanced assembly. Most commonly the assemblies are balanced and not the individual parts.