2003 MN6 Convertible ~30,000 miles.

Recently I noticed that my harmonic balancer had the classic wobble so I decided to replace it. After replacement I noticed a very distinct engine vibration at ~1700 RPM. This vibration occurred with clutch in/out, car at speed or parked, accessory belts on or off, engine loaded or unloaded. I checked the engine & trans mounts and they all seemed OK. All signs pointed to the harmonic balancer, even though it was brand new and I followed the GM service manual to the letter....or did I? I decided to investigate the possibility that my new harmonic balancer was imbalanced OR that I somehow missed a balance weight installed on the old harmonic balancer.

For those not up to speed, sometimes weights are added to harmonic balancers (manual transmissions only) at the factory. When replacing a harmonic balancer you must mark its position relative to the crank shaft and ensure the weight is transferred from the OLD to NEW at the correct orientation. These weights in combination with flywheel weights ensure the rotating engine mass is properly balanced.

Unfortunately I threw out my old harmonic balancer so I couldn't go back and check for weights that I may have missed. My new harmonic balancer had 12 equally spaced holes drilled in the face of the "pulley" section, 1" deep and 1/4" diameter. I marked these holes A to L and rolled a suitability sized lead plug (1" long - 1/4" dia). One-by-one I test fit the lead plug into each of these holes, starting the car each time and revving the engine to ~1700 RPM. At hole "C" the vibration completely disappeared and the engine was 100% vibration free - smooth as glass. To finalize the installation of the plug I simply used a metal punch to deform the lead into position so it will not work itself out over time.

I'm not sure if I overlooked a weight installed in my factory harmonic balancer OR if the aftermarket harmonic balancer was not balanced. Either way I hope this will help anyone changing their harmonic balancer OR troubleshooting an engine vibration.

This is my first Corvette and this Forum has been a huge help in all my repairs & maintenance.

 

Humm ...interesting

 

Thank you. i'm about to remove my balancer.
I wonder what happens if you replace with an aftermarket balancer?
This just seems odd as the LS is supposed to be internally balanced.
You might consider an epoxy to cover your lead slug just for security.

Ron

 

Ron, The LS engines are internally balanced. However, on manual transmission Corvettes, they are sensitive to any imbalance due to the unique drivetrain. Because of this, each engine for the manual transmission Corvettes go through a hot balance at the end of the assembly line. All other small block applications can use what is called net build engines without any issues. By carefully controlling the mass of each rotating component the imbalance can be kept to less than 1.5 in oz. The manual Corvette, C5 & C6 need the imbalance to be less than .5 in oz.

The older top post above is an example of a manual balance. The key to success is using the same steady-state rpm point and feeling the vibration at the same place, like at the shifter.

 

Ron, as a precaution I peened the circumference of the harmonic balancer weight hole so that the lead would not work its way out.

With regards to your question about aftermarket harmonic balancers; they do work but you have to manually balance them once installed.

 

Reading this discussion has me confused, somewhat, as I'm considering replacing my balancer with an aftermarket unit, such as an ATI.

I have a GM shop manual, and recall reading the procedure about removing the "weights" from the OE balancer, and transferring them to the new balancer. This, however, assumes that you're installing another GM balancer. Does this mean that if I were to install an ATI balancer, I might have a balance issue with my engine?

 

The simple answer, Maybe.

First thing is to check the original harmonic balancer to see if it has weights in any of the 12 holes. If not, then just replace it, you should be good. Most of the aftermarkets are not setup with the balancing holes like the stock unit has. In my opinion if you find one weight in the original I wouldn't do anything special, just install the new one and don't worry about the imbalance. The thing to keep in mind, the small amount of imbalance will not hurt anything. There might be a slight increase in vibration but not enough that most folks will even notice.

If the balancer has more that two weights at two adjoining holes out of the 12, you may want to match balance. This is assuming that the original balancer is still in good shape. The harmonic balancer has to be indexed to the crankshaft to keep the proper orientation for the match balance. Most any good machine shop can match balance for you.

Again, a lot of folks don't bother worrying about the imbalance and just install the new one. It won't hurt anything not to balance.

 

Thanks, Eric.

 

As a followup, a fishing company named Bullet Weights sells a 1 pound coil of 1/4" solid core lead for around $6. A pound is much more than you need but the 1/4" diameter size is a perfect fit.

 

Interesting, and a good tip.






With respect to replacing the balancer, since this is an old discussion, I went ahead and replaced my balancer (eventually, in 2013) with an ATI piece. I did not have any issues with vibration.

 

Replaced my balancer with a Powerbond and ARP bolt. No vibration, no balancing required.

 

Being technically/mechanically slow, I was taken aback when my trusted mechanic today told me I would be paying $660 for replacement of a harmonic balancer & serpentine belt on my 2000 A4. "Labor intensive" was the phrase. Now I see the truth in that. I appreciate this discussion greatly.

 

yes it is. not hard but time consuming and greasy.

 

thanks for the post, as an owner of a stick shift, I will keep this information in mind when new balancer time comes around.