flywheel lightening; accuracy of lathing machines

i just recently had my flywheel lightened by a machine shop. one of the tips that i got from our local mechanics was that I had to have the lightened flywheel balanced afterwards. but most of the people I asked said that it did not require additional balancing because it already came from a lathing machine. i was wondering whether anyone could help me clear my mind of paranoia from possibly damaging my bearings. also, what are the earliest symptoms that the lightened flywheel is not properly balanced?

I dont see where people are getting that it doesnt need to be ballenced because it was a on a lathe. If you put your hand on the motor and there is noticable vibration then it is probably out of ballance. Rarely is the rotating mass of an engine completely ballanced, thats why most engines use harmonic ballancers, and even then some motors are notorious for having vibration issues.

The balancers many engines are equipped with is for the reduction of torsional vibration, that's something completely different.

I would recommend that you at least check the balance after the flywheel has been machined. Billet crankshafts are machined on lathes and milling machines, but in the end you balance those too.

i just recently had my flywheel lightened by a machine shop. one of the tips that i got from our local mechanics was that I had to have the lightened flywheel balanced afterwards. but most of the people I asked said that it did not require additional balancing because it already came from a lathing machine. i was wondering whether anyone could help me clear my mind of paranoia from possibly damaging my bearings. also, what are the earliest symptoms that the lightened flywheel is not properly balanced?

A lightened flywheel is a interesting concept. How much was removed and at what radius? Anyways the machine shop that did the job should be the one to tell you if material was removed in a "balanced" way. If so then the balance is the same as it was before. If not then the balance has changed (probably for the worse) and either way it should be checked as state above.

If it is a wheel with heavy spot on purpose then it will take otherr engine components to check it. If it happens to be a "zero balance" wheel then it can be checked by itself.

There are two kinds of balance ---- static and dynamic. Just because you remove metal using a lathe doesn't mean that both balances are acheived. Then there is the situation of how the flywheel was chucked in order to take the cuts. And then, I'm sure that the machinist can "wash his hands" of any problems if he tells you to have it balanced afterward...
In any case, it's probably not a bad idea to check balance since it's out of the car especially if you have a higher revving engine. Taking away mass from the flywheel will probably make the engine run less smooth anyway so every little bit helps.

All good points... and to add another; just because the lathe removed equal amounts of metal all around doesn't mean it removed equal WEIGHT from all around. Differences in casting homology and porosity in the flywheel can make significant differences in final balance. I'd have it rebalanced.

how much weight came off when you got your flywheel lathed? and where do they take it off? This just sounds kind of dangerous to me...I dunno. I guess it would depend on how much came off. Does it seem that way to anyone else?

There's a chance it came out of the lightening process completely balanced.

But there's a better chance that it didn't.
I'd get the balance at least checked.

how much weight came off when you got your flywheel lathed? and where do they take it off? This just sounds kind of dangerous to me...I dunno. I guess it would depend on how much came off. Does it seem that way to anyone else?

It may be dangerous depending upon the removal, but mostly seems like a waste to me. A flywheel is heavy for a reason. I've heard of light flywheels and used them a lot, but never have lightened one.