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Please Explain SBF Engine Balance To Me

B

bravenrace

Member
Okay, now I understand about the 28oz and 50oz imbalance (I think), and I fully understand internal engine balance, but I'm having a problem that has led me to question some things.
I converted from an automatic to a T-5 this past winter. My '65 currently has the original 289 in it, so I bought a 28oz flywheel. I made no changes to the engine at all.
Now I have a vibration that increases with RPM, regardless as to whether or not the clutch is engaged. All the conversion parts are brand new, with the exception of the T-5
bellhousing.
Here's the thing. If the flywheel has a 28oz imbalance, why doesn't it have a dowel pin or some other means of indexing it? Installing it in any position would seem to be similar to putting a wheel weight in the wrong place. Or should it be indexed? I think I'm missing something when it comes to understanding the way the SBF is balanced, and every source I can find talks only about the differences between the 28 and 50oz engines instead of explaining the theory of how the engine is balanced. Can anyone shed any light on this for me?
Thanks,
Jim
 
The flywheel should be independently balanced first. The pistons and rods are then all balanced. Then the flywheel, the crank, damper rods and pistons are balanced as a rotating assembly.

Your flywheel does not need to be indexed for balance purposes.
 
Okay, but why? It has weight added to it in one particular place, so why does it not matter where that weight is applied in relation to the rest of the rotating assembly?
 
The flywheel only bolts on one way. The holes are not symmetrical in that they is not equal spacing between holes
 
"bravenrace" said:
Okay, but why? It has weight added to it in one particular place, so why does it not matter where that weight is applied in relation to the rest of the rotating assembly?
Click to expand...
It's round, right? If it is balanced it doesn't matter at which degree it gets affixed to the crank as it is irrelevent. Do you have to put a balanced tire on in any particular postion? No. These things are round...and balanced.
 
I thought the flex plate/fly wheel could only be installed one way. Aren't the bolts spaced slightly different so that all of them cannot be installed without it being properly positioned? If so that would enable the weight to be installed in the same position every time.
 
"Horseplay" said:
It's round, right? If it is balanced it doesn't matter at which degree it gets affixed to the crank as it is irrelevent. Do you have to put a balanced tire on in any particular postion? No. These things are round...and balanced.
Click to expand...

The flywheel has a 28 oz. imbalance. If it was just a neutral balance, then they wouldn't have two different flywheels based on the 28 or 50oz imbalance.
 
"bravenrace" said:
The flywheel has a 28 oz. imbalance. If it was just a neutral balance, then they wouldn't have two different flywheels based on the 28 or 50oz imbalance.
Click to expand...

again, as stated above, it only bolts on one way. The machine shops balances it to the bolt pattern. It is not neutral balanced and you are correct on the two different balances ie 28 and 50oz. If you have one handy take a look where it bolts to the crank and all will be clear.
 
"bravenrace" said:
The flywheel has a 28 oz. imbalance. If it was just a neutral balance, then they wouldn't have two different flywheels based on the 28 or 50oz imbalance.
Click to expand...
time for me to shut up now. :roul

I guess the one little fact that it does only align in one way is the missing piece I forgot.
 
"kb3" said:
again, as stated above, it only bolts on one way. The machine shops balances it to the bolt pattern. It is not neutral balanced and you are correct on the two different balances ie 28 and 50oz. If you have one handy take a look where it bolts to the crank and all will be clear.
Click to expand...


As stated, the flywheel only bolts up one way!
 
Is the difference in the bolt pattern minimal? I have a picture of mine installed and the bolts to my eye look symmetrical.

Also, just so I'm clear, if I buy a flywheel that is advertised to be for a 28oz imbalance, and that is correct for my engine, I should be able to just bolt it on and go?
If so, I'm back to square one on my vibration.
 
Does the vibration occurs when driving or when you just revs the engine stationary ??
Harmonic damper ??

The flywheel can only be mounted one way . One mounting hole in the chrankshaft has a small offset.
As stated by most of us old dudes !!
 
It does it with the clutch engaged or disengaged, moving or not. Nothing on the engine was touched except the flywheel, so while it could be the balancer, it's unlikely.
 
I too did make this conversion.
When I bought my flywheel , it was delivered with both weights. 28 and 50 oz so I had to bolt the correct weight to the flywheel. :confu
Is yours bolted to the flywheel too ??? :confu :confu
 
Did you use a pilot bushing that is mounted in the center of the chrankshaft ?
 
I don't remember for sure, but I am pretty sure it was wrapped in an air tight shrink wrap. I imagine what you are getting at is that it may have been packaged incorrectly. I'm thinking it's certainly possible but not likely, although if I can't figure this out I'll have no choice but to pull the trans and check everything.
When you guys say the flywheel can only go on one way, are you saying that there is no way possible that it couldn't be off? Looking at the picture of my installed flywheel, the holes must not be off that much, so I'm wondering if there's enough hole clearance that someone could install it wrong if they weren't careful or if there's just no way to do it wrong.
 
IMO you can't mount it incorrectly.
You should have noticed that one of the bolts hits one side of a mounting hole if it isn't align properly.
With some force it could be done !?!?I just don't know because I know it and look for the perfect alignment.
The holes should align perfectly with the ones of the crankshaft.And all the bolts should be screwed in by hand unless the bolts are prelubed with some kind of loctite. :confu

Can or did you see the weight on the flywheel ?
 
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