What Greg Fast said is correct, I don't do it anymore because for me it's a job that really is waste of my time and taxes my lathe for the money it nets, no offense, but I have bigger fish to fry. It's a worthwhile thing to do (flywheel lightening) as part of a bigger picture, meaning if you'e going to build a performance engine and are doing many things, then doing this is a good thing to do as part of that package. I'll probably end up doing it again, when I get time to put a new motor on my lathe, I think my motor just got old and weak and this is why the failure happen, but only for the customer that gets whole burrito, meaning I'm building you a performance engine.
I don't lighten V8 cranks on my lathe anymore either, plain and simple the guys don't want to pay the worth of the job. It's nothing persoanl it just the way I do it, it just not a well enough paying job for the time invested, I make better money doing something else, and the light bill does come every month :)
Ok, now I would like to talk about engine balancing. Tom B made a comment about getting your bottom end components balanced together. I don't think he actually meant together, but it is a common practice with some shops, I elaborate more on this. Sometimes a engine builder/machinist will balance mutiple components at one time, this is a very bad idea, easier for the machinist, maybe cheaper for you, but a bad idea overall. We practice something called zero balance and have for some time, meaning everything is balanced and balanced separately. Example, say you want to have your crank, flywheel and harmonic balancer all balanced and the machinist tells you he will bolt all three together and balance that way, then for him its quicker, but for you it could be problem in the future if any of those three items are taken out of the equation. First off a inline motor like ours does not have to simulate rod weights with bob weights, just V motors have to do this. Ok say you have the three balanced together, the machinist can remove material from any of the three components to acheive overall balance in those items, but say a year down the road your harmonic balancer rubber takes a crap for example and you need to replace it, you're screwed, because removing any of the three items balanced together out of the equation throws everything off and you could end up being worse of than factory balnceing to begine with, so that's why balancing things together is a bad idea.
Balancing your flywheel, harmonic balancer or crankshaft all require a rotating balancing machine, balancing your pistons and rods is a static balance meaning they are not rotated, but pistons and rods are static balanced quite differently. With the piston, just put them on a scale that measures in atlaeast in grams or even tenths of grams, just get them to all weight as close as you can to each other, now rods are totally different ballgame and are not for most backyard guys, you need a rod trapeese, you have three things on connecting rod to balance, top end weight, bottom end weight and overall weight, if you're not experienced in doing this or don't have the right equipment, then you could do more harm than good, never, ever just overall static balance your rods, even if they weight all the same overall you may have top or botom weights that get to the same overall weight very differently and end up being way worse than factory balancing. Balancing engine components is a good thing overall, it comes into play in a race motor as a necessity because of the high constant RPMs, but street engine that won't be zinging along above 6 grand all the time can benefit as well, although not a necessity, most folks comments on a fully balanced engine will be that it feels like a much smoother running engine. I once read here where someone said the factory does not balance your engine compnents, that's hogwash, they balance everything, just not too as tight of a spec as you can, for exmaple if I had to guess what factory balance spec were on a MGB crank from doing them, I would say three grams or less, when I balance your crank, I normally get within a half of a gram, so all you are doing is tightening the specs.
OK, last but not least, any of you that have taken the time to read book like Vizard's books, will know lightening any rotating component will not net you even one single horsepower, but it will increase your accelaration rate. In the race engine we get everything down as light as we can reliably, we lighten rods or buy a aftermarket rods like a Carrillo, we wedge lighten the cranks, we pay around with custom piston design that may run lower compression heights, and use smaller diameter pins and different pin metal type to overall lessen the piston/pin weight. I just designed a MGB race piston to run a .750" pin instead of the .812" pin, pins are heaviest part of this component, going with a smaller diameter taper bored pin could easily lower the overall weight a good 30-50 grams per piston, that's alot in the overall weight of a piston/pin. Take two identical race cars with engines prepared the same, say both weight exactly the same and both have the same exact horsepower, but one has spent alot of time and money to lower it's engine's rotating mass, the lighter rotating massed engine will accelerate quicker, given a straight is a given length, then the lighter rotating mass car will reach maxiumum velocity quicker and cover the same distance in a shorter amount of time. The lighter rotating mass car will always under the same exact conditons exit a corner quicker than the car with more rotating mass. This can go on to alot more than just internal engine components with things like extreme light weight wheels etc.
Here's a picture of 1275 wedge lighten racing crankshaft I did, we remove around three pounds off this crank, we done alot of these over the years. The crankshaft is on a Hines computerized rotating balancing machine, I get to use this machine with the machine shop that does my boring and line boring. This is by far the nicest balancing machine I ever used, fiquire if you had to buy this machine brand new you'd be looking at aprox. $60K. I would never live long enough to break even if I owned one of these machine, so lucky for me, these guys rent me the use of this one.