When I had my rotating assembly I supplied a 153 tooth flywheel.
I am swapping the engine into a truck which uses a different clutch and a 168 tooth flywheel.

It is a one piece rear main block, with an external balance flywheel. When I picked it up, the machinest said if I ever changed the flywheel, make sure I use the same type.

So, can I simply swap a 168tooth external balance flywheel?

As long as they are both external balance thru should be the same right?
If he said I could even change it, then I would assume he did not change the weight on the flywheel.

If the rotating assy was set up properly for external balance, any ext replacement ext balance flywheel should work with no issues....

It was balanced with the external flywheel.

I was just wanting to make sure.
I didn't think it would be common practice to alter the weight on the flywheel itself, nut figured I'd double check.

Unfortunately, it's ALL TOO COMMON.

Take a look at it and see.

If it's been TOUCHED, then you're hosed.

THIS RIGHT HERE is the reason I post over and over again, DO NOT UNDER ANY CIRCUMSTANCES EVER allow your rotating assembly to exist in the same shop at the same time with your externals!!! Rather, hand them your rot assy, and tell them MAKE ITS BALANCE STOCK; then later on (or vice-versa) take them your externals, and tell them to trim them up to exactly STOCK balance. That way, you preserve your interchangeability for all time with all parts.

If a shop tells me they "can't" balance my rot assy without the other things, I tell em to pack it up WITHOUT TOUCHING IT ANY FURTHER and I'll be by ASAP to get it; and that I will NEVER AS LONG AS I LIVE EVEN IF THEY'RE THE LAST BALANCE PLACE ON THE FACE OF THE PLAENT, bring them another piece of work. I will balance it myself with a lawn mower blade balancer before I'll give money to somebody that would deliberately sodomize a paying customer that way.

Few things are more dishonest IMO than a balance shop "balancing" your parts by altering the externals to match an IMPERFECTLY balanced rot assy, rather than getting the rot assy RIGHT in the first place. I hope you haven't been the victim of an unscrupulous lazy person taking advantage of you.

Can you tell, I feel rather strongly about this?

I know they took a lot of material off the crank counterwieghts.

This is a reputable machine shop that has been around for many years building justly race assemblies.

Also, as I mentioned he said" if I ever replaced the flywheel to use the same type."
Not " if you ever replace the flywheel
it will need rebalanced"

This may sound crazy, but when a one piece rear main seal crank flywheel is replaced, there is a GM procedure that is used to balance in the new flywheel. It involves weight being placed into the outer holes of the flywheel.

I posted it here someplace, let me see if I can find it.

It doesn't make a lot of sense as flywheels with offset weights have been being made for many years.

RBob.

Here is the thread with the balancing info, right from the '92 FSM:

https://www.thirdgen.org/forums/tech...balancing.html

RBob.

Given what he now has, could he not get is new 168 tooth flywheel " unbalanced" to match the balance on his modified 153 tooth version?

IF the engine has the stock balance, you should be able to use a stock truck flywheel.

The 305/350 1-pc rear main seal is internally balanced, same as earlier-model SBCs. Only thing is, the rearmost internal balance weight, which would have been on the funky-shaped flange of the older motors, had to be moved to the flywheel, since the flange had to become round for the seal. The motor is still internally balanced; the internal weight has merely been moved to an "external" looking location. This DOES NOT make those motors "externally" balanced. (even though some aftermarket mfrs *cough Eagle cough*) have given up on the TRUTH and caved to the slang and misconceptions of the ignorant masses.

But however all that may be, "stock" is the operative concept. If your existing flywheel is STOCK balance, then your new one can be, as well.

And yes, you should be able to get any other flywheel matched to your existing one, as well; not quite as sure a thing as "stock", but a possible path out of the weeds, in case you're in them.

It was balanced at a machine shop using a factory replacement harmonic dampner and replacement flywheel both parts listed for a 1988 engine.

No flywheel or flexplate is perfectly balanced or identical to another.

When I had my engine balanced, I provided a neutral balanced SFI flexplate for my internally balanced engine. The difference with and without the flexplate was 9 grams. Might not sound like much but 9 grams gets heavy at 7000+ rpm.

My downside is the SFI flexplate is only good for 3 years from the date code. When I replace it, I don't strip my engine down to get it all rebalanced with a new flexplate.

Your 153 vs 168 tooth flywheel will not be close to the balance that the rotating assembly was balanced for. Your machinist said the same type. You're going to a larger flywheel which is not the same type. When you feel a vibration after the swap, you'll know the rotating assembly will need to be rebalanced for the larger flywheel.

Very few production engines are perfectly balanced from the factory. With the engine in a truck, there probably won't be much of an issue. Watch the antenna vibrate if it has an external antenna.

xxx oz-inches at yy ° = xxx oz-inches at yy °.

Doesn't matter who made it, what the overall diameter is, whatever whatever whatever whatever; the principle is too simple. It's eeeeeezy to outsmart yourself.

xxx oz-inches at yy ° = xxx oz-inches at yy °. ALWAYS. Take it to the bank.

Either it's BALANCED (within some acceptable tolerance of oz-in at some acceptable azimuth in °), or it's NOT.

This stuff is just not that hard. It's just a car. The laws of physics aren't nullified just because it has 4 wheels.

Alky is right, factory motors are RARELY "perfect". What they ARE, is within a band of "acceptable". All that has to be done now, is duplicate that; and you'll be fine.

Which as long as you can somehow figure out what you've got NOW, if it's OK, then if you duplicate it, that'll be OK too. It's the figuring out part that might (or might not) present difficulties.

All you gotta figure out, is how many oz-in, at how many °. Might be trivial, might be like pulling teeth, to get at that; but if you strip away all the "car" mythology and folklore, and concentrate on the scientific physical principles, it's a solvable problem. (PITA factor notwithstanding)

Yes, if it was balanced properly within the acceptable "tolerances"…which may be a wide range as Sofa is saying…then you should be fine using another flywheel from another externally balanced engine. It's not like a flywheel isn't a wear item……the diameter of the flywheel shouldn't matter either, so long as it was balanced within "tolerance" for an externally balanced engine.

Yes, they have been made for quite a long time….

Based on that, can't Johnny take his current 153 tooth flywheel to be measured for it's specific "xxx oz-inches at y°". Then, bolt the 168t flywheel to the same balance fixture, and make any weight adjustments, only on the flywheel, to make the 168t measurement match the 153t measurement? That would make the two flywheels have identical balances.

Sure, the car will be down for a couple hours or days. But does not require the engine to be torn down.

Yes: you can actually take the old one and the new one, stick them together back-to-back such that they are exactly 180° opposite each other, and then balance the new one to "neutral" the pair.

This is probably what Johnny will need to do.

You could theoretically do this in your own garage with a lawn mower blade balancer approach if you had to, though I certainly don't "recommend" that.

Flywheels aren't too awful bad to deal with in such a situation, when you have the old one. They're flat and easy to handle, easy to make sure they're arranged exactly as you think.

Where you REALLY get hosed though, is when you no longer have the flywheel; or, something internal to the motor needs to be changed.

Just to update, I installed an out of the box 168 tooth flywheel.

Got over 1000 miles on it so far. No vibrations. Runs great!

I'm in a similar boat, however I'm pretty sure I will need to have mine re-balanced.

I purchased a short block assembly that was balanced that has a couple of bad pistons. Was able to clean the cylinder walls and not need additional boring, it's already at .040. The Lunati crank was balanced with a flex plate, I'm using a flywheel. I don't see the "batwing" on the flywheel, so I'm supposing they nuetral balanced the rotating assembly.
It's a 6" rod 3.75 stroke 383, when I replace the pistons I guess I'll need to have the whole assembly rebalanced with the flywheel. Just don't know if I need a "regular" 1piece rms flywheel or a "special" neutral balance flywheel.....
This is a long term project so I've plenty of time to think about it, but it's hard trying to figure out what another machine shop did....
The previous owner said "I dunno, I just had it balanced"....