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When I had my 47 Clipper rebuilt I had it balanced. The machine shop told me when I picked it up that it was so close to being perfectly balanced from the factory that they would not waste their time balancing another.

"Balance a nickel" type of smoothness at idle has to do with even compression, good ignition and carburetion. The benefits of balancing won't hardly be noticed til higher revs.

My 51 200 has less than perfect compression and is somewhat lumpy at idle. Yet if I wind her out to 45 in second, she is smooth as a turbine--underlining that the factory balance was indeed quite good.

When rebuilding straight eights I will weigh a new set of pistons to be sure they are all the same--and they always are within a few grams. Inconsequential.

On inline engines the crank is balanced by itself, and the weight of the pistons is not important as long as they are all the same.

Vee type engines are a different matter and the weight of the pistons and rods must be accounted for in the balance of the crank. Therefore if non standard weight pistons are used the crank must be rebalanced.

I love inline engines.

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...and crankshaft bearings, valve clearance, engine mountings, ...

Running smoothness has many fathers and provides information about the system. There is no "balance job" - to balance an engine means to make a good rebuild job. It isn't a separate operation but a resulting property.

What is most important is to make sure the crankshaft is align bored with the engine block. If you don't do it, there is a very high chance that the bearings will wear out in very short order. Learned the hard way on this on more than one engine.

West, not quite sure what you mean by align boring the crankshaft; on an engine with insert style main bearings I suspect you mean align-boring the main bearing saddles with caps installed to insure they have a common centerline? That's a common proceedure by the better engine rebuilder, especially with "V" type engines.