Hey guys I just recently got a new lightweight aluminum flywheel installed into my 1996 LT4, it's the Fidanza one. My trusted mechanic called me (has run a corvette shop for 45 years) and explained to me that the process for replacing the flywheels was he would have to match the weight to the OEM flywheel that was in the before, the only thing is the previous owner of my car put in an aftermarket flywheel already and didn't have it balanced. So he said he couldn't really do anything and that he was just going to put it in straight up. Now even though my clutch feels great, there's this constant terrible sound at super low rpms in high gears or just when I'm accelerating. Can anyone let me know how I should approach this? I asked him if the weight was just information he could find online because I thought this had to set number people knew but he said no. Help would be appreciated! How am I supposed to go fast with a smile on my face when all I can hear is her screaming at me.. and not in a good way!xD

 

Let’s first be sure what you are dealing with.

The Fidanza did not need to be match balanced as long as it came pre “counterbalanced” for an externally balanced SBC. If it had balance weights on it, it should be fine.

An improperly balanced FW should cause a vibration when revving the motor and likely not at lower RPM.

Is this just a sound or is the car shaking?

what happens if you depress the clutch when this happens?

Does it only happen when the car is moving or can you replicate with clutch fully depressed and revving the engine (if that is smooth your issue has nothing to do with flywheel)?

Could be excessive gear rattle, bad pilot bearing or trans to engine alignment among other issues…

Also please describe the sound more: whirring? Rocks in a can/rattle sound? Growl?

 

Car doesn't shake, just the sound and only happens mainly in super high rpms, so I haven't been bringing her up there while she's in this condition.
The sound occurs when the clutch is depressed, if I press the clutch in the sound goes away. I know this by when it's idling you can hear it and if you just press in the clutch it goes silent. The sound is sort've high-pitched, super harsh, rocks in a can/rattle sound is a good description of it.

So you believe it should've already came pre weighted? And that this noise might be the outcome of another problem?

Thanks for the response!

 

If the correct Fidanza was bought for your car it should have come with counterbalance weights installed (that’s how Fidanza does it). If the flywheel was incorrectly neutral balanced your engine would shake pretty violently..so that is doubtful.

In your first description you stated sound at *low* RPM and “high gear”, not “high RPM”. Can you clarify?

You state it sounds like “rocks in a can” and at idle you can hear it, and pushing in clutch pedal (ie disengaging the clutch) it goes away?

That is classic “gear rollover” noise that happens with a SMF (single mass flywheel). The lighter the flywheel (you went aluminum so very light) the more likely it will happen. You will also hear a “growl” if accelerating/loading the engine when the engine is at low RPM..that may also match your high gear low RPM description.

Good news is “gear rollover” noise is completely harmless. It is your ZF6 countershaft having play (normal from factory). This is why GM put DMFs in (dual mass flywheels) as they dampened the engine vibrations from reaching the transmission.

There is a countershaft shim that can be installed on the transmission that reduces or may eliminate this entirely, but requires transmission removal to install.

This would be by far most apparent at idle, trans in neutral and foot completely off the clutch pedal.

Does what is above sound like what’s happening?

Can you confirm that with clutch pedal to the floor and revving there is no sound or unusual vibration?

Only other thoughts are pilot bearing noise (either bad pilot or misalignment of trans) but that would be more of a whirring sound.

I would suggest giving Bill at zfdoc a call too for a double check and reassurance.

 

The one piece RMS Gen I or II engines with factory balance specs are zero balanced at the front balancer, and 23.4 oz-in external balance at the rear. Fidanza Billet Aluminum Flywheel 198501 can come either with or without the proper balance weight, once zero balanced if you want to take it to a shop and have it balanced, then weight (5.2oz @ 4.50") added (kit 337775) is designed for this. Been there, done it, great results.

The "use your old flywheel and balance new one to it" solution works, depending on how well it was balanced in the first place.

 

Yep bingo…and the stock Fidanza or other quality flywheel that comes counterbalanced should be fine and don’t require match balancing…especially with a worn DMF or if already converted to SMF match balancing doesn’t make a lot of sense.

Hopefully the OP’s issues are just rollover noise based on the description and the mechanic didn’t goober something….

 

I agree with all the replies. What would bother me, though, is that the OP is getting the rollover noise at high RPMs as well as at idle. It makes me wonder if the mechanic installed a solid clutch disk, which will make the rollover noise worse. The stock dual-mass flywheel setup in the ZF6 cars came with a solid disk, but it relied on the "floating" mass of the flywheel to soak up vibrations and harmonics. When people switch to a single-mass aftermarket flywheel, they need to use a sprung-hub disk like the one that came in F-bodies of the same era. If they don't, the transmission will make a hell of a racket.

 

Good point, I had the same thought too…but that’s why I asked for clarification because at first it was described as “high gear low rpm” and then “high rpm”…. It’s hard to tell from here without more detail…. I was also wondering this because the OP already had a SMF in the car…yeah went lighter BUT I bet the OP would have at least partially heard the gear rattle before…. That to me is the most concerning.

with a non sprung disk, it would likely be harsh/shudder when releasing the clutch I would think…. If @1Eyed Willie sees this he may be able to describe what a non sprung with SMF sounds/feels like…

 

Heard it rattle before but it was worse than it is now. The flywheel that was installed before was damaged also so I thought the sound might've been because of that.

Didn't check the package to see if it came with counterweights or not I just gave him the box unopened. But yes my engine does not shake violenty so I think its ok there.

yes I stated at "low rpms" as in around 2k as im accelerating and quiets down a lot around 3k but can still be heard. Doesn't matter what gear it is in, can be heard in all. Sorry if my phrasing was confusing, im still getting used to using car lingo in sentences lol.

The "gear rollover" sounds like it would be the best explanation for this, I also found a YouTube video of a guy showing the same noise off that's in my car. I'll link it so you can get a better idea of the sound without having to rely on my words ahah.


I can confirm there is no noise when clutch pedal is to the floor and revving with also no vibration.

Do you have an idea of where I can find this countershaft shim for purchase?

Thanks again for the response, you've been super helpful.

 

Mechanic is closed this weekend but I'll call in on Monday to see if he installed a solid clutch disk. Would be a pain if he did because no way am I paying for labor again, I'll have to do it myself. But that's good information to know, thank you.

 

I honestly can't hear the sound very well in that video over the exhaust. That's a good thing. I think if it's only at idle and low RPM and it goes away when you disengage the clutch, then it's almost certainly rollover noise. It may be about the normal amount for a ZF6 with light single-mass flywheel and the correct sprung-hub disk. I had the same flywheel and sprung-hub disk and mine rattle at idle but generally not when in gear. So the main thing you need to do is just verify that the correct sprung-hub disk is in there.

The noise is normal for a ZF6 (and some other transmissions to a lesser extent). It doesn't mean any harm is being done to any parts, even if you have a solid-hub disk in there. So there's no emergence to fix anything. The countershaft shims were available from Carolina Clutch for roughly $50 last time I looked. Bill Boudreux - the "ZF Doc" - probably sells them, too. But you'd have to drop the transmission (but not the clutch) to install those, too, since they go in from the front. The shim kit would be something to do if you end up dropping it to replace the disk. If you end up doing this yourself, it's not rocket science but it is a fair amount of work.

 

Agreed on pulling the clutch being a “fair amount of work” . I have a thread documenting my journey. Very doable though.

OP if you can post the part numbers of what’s on there you/us can check to see if what was installed is correct. A solid disk might explain how much you are hearing. I’m assuming the trans fluid was fully changed, what oil went in it?

How persistent your rattle is and at 2k rpm is a lot more than mine. However I went steel flywheel and put the countershaft shim in.

 

Ok, i have a 1993 and went from stock dual mass to lightweight alum fidanza flywheel.

Your noise (esp with clutch pedal out) sounds similar but worse than what I had.

the noise isnt coming from the flywheel.
its coming from the trans and the new flywheel isnt doing a good job masking it (which is normal for the alum flywheel)

you need to fix the noise at its source which is the trans.
shim the countershaft. Zfdoc has the information you need to fix it

He had me take some measurements (of the existing shims) and then prescribed a thicker shim (cant remember how much thicker, it was in thousandths of an inch)

this cut 90% of that noise out,