C6 REBUILD VIBRATION

C6 REBUILD VIBRATION

Joined: August 17th, 2011, 1:13 am

August 17th, 2011, 1:55 am #1

I had the C6 on my '66 Fairlane GTA rebuilt as it wasn't engaging when shifting. The garage doing the work came highly recommended by other muscle car owners and has a solid reputation. The flexplate needed replacing as it was missing a tooth and had other wear on it. The engine is a 428CJ. Prior it ran strong and smooth. When they rebuilt the trans and installed the new torque converter they told me that the trans had been done before but "it only had 5 stages, instead of 6 stages, plus some/all springs were left out." "Probably to soup up the trans and make it shift tighter." The new flex plate they put in matched what they took out. The car runs strong and shifts at speeds/rpm where appropiate.

My problem is that when running at 3,000 rpm it developes a high frequency vibration. I took it back and they rotated the drive shaft 180*. That change it very little, if at all. I over inflated the tires, and took it out on a smooth newly asphalt paved road driving over 70mph and put in neutral, and let off the gas. The vibration went away. This was done serveral times with engine/trans fully warmed up. So it appears that it is RPM related and mostly coming from the front, through the steering wheel, gas pedal, and floorboards.

The question is what to do next to find the cause and eliminate the vibration?
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Joined: July 17th, 2011, 7:07 pm

August 17th, 2011, 2:41 am #2


Is your 428 still externally balanced? (not been converted to internal balance) 

The shop may have checked/compared the flex plates, but I would have wanted to look at them personally, just to verify. (I'm anal that way)

It's not unheard of for a torque convertor to be out of balance.

Do you experience the vibration if you just smoothly bring the engine RPM up to 3000 in park? If so, then it's obviously engine/flex plate. or convertor related.
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Joined: November 18th, 2004, 4:20 am

August 17th, 2011, 1:41 pm #3

I had the C6 on my '66 Fairlane GTA rebuilt as it wasn't engaging when shifting. The garage doing the work came highly recommended by other muscle car owners and has a solid reputation. The flexplate needed replacing as it was missing a tooth and had other wear on it. The engine is a 428CJ. Prior it ran strong and smooth. When they rebuilt the trans and installed the new torque converter they told me that the trans had been done before but "it only had 5 stages, instead of 6 stages, plus some/all springs were left out." "Probably to soup up the trans and make it shift tighter." The new flex plate they put in matched what they took out. The car runs strong and shifts at speeds/rpm where appropiate.

My problem is that when running at 3,000 rpm it developes a high frequency vibration. I took it back and they rotated the drive shaft 180*. That change it very little, if at all. I over inflated the tires, and took it out on a smooth newly asphalt paved road driving over 70mph and put in neutral, and let off the gas. The vibration went away. This was done serveral times with engine/trans fully warmed up. So it appears that it is RPM related and mostly coming from the front, through the steering wheel, gas pedal, and floorboards.

The question is what to do next to find the cause and eliminate the vibration?
I could be mistaken but I think some 428's had a thick plate attached to the flexplate and convertor.

Sort of evened the vibrations.


Yup, I just looked.

428 FLYWHEEL C9ZZ-6375-B (Manual Trans.) or C9OZ-6375-B (Auto. Trans.), 428 VIBRATION DAMPER C8AZ-6316-C, and VIBRATION DAMPER SPACER (with counter-weight) C9ZZ-6359-A.
Last edited by stuart40a on August 17th, 2011, 1:43 pm, edited 1 time in total.
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Joined: July 17th, 2011, 7:07 pm

August 17th, 2011, 6:14 pm #4


Yes, that's what I meant by externally balanced. I forgot to say to look for the weight welded to the flexplate.

Should be able to see the weight thru the front inspection cover, may have to rotate the crank to see it.

I believe the weight should be on the front side of the flexplate.
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Joined: November 18th, 2004, 4:20 am

August 17th, 2011, 8:36 pm #5

It's not a weight weight, it's a counter balance weight.

Big enough to bolt onto the flex plate.

It is at least 10" across.
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Joined: August 17th, 2011, 1:13 am

August 18th, 2011, 1:29 am #6

Yes, that's what I meant by externally balanced. I forgot to say to look for the weight welded to the flexplate.

Should be able to see the weight thru the front inspection cover, may have to rotate the crank to see it.

I believe the weight should be on the front side of the flexplate.
The flex plate has a weight welded near the outer rim as shown in the link below. I saw the one that came out and it matched the photo on the left in http://www.428cobrajet.org/id-flexplate.html . I did not see what went in, but they assure me that it matched. ( They called me when they got a flex plate to install as I had told them the motor was a 428, and they asked if the motor was a CJ? I had left this detail out. The first flex plate they got did not match what was on the engine. When the got the correct one they said it matched.) There were no other plates, balance parts or bolt-on items.

I have run the engine in park to high RPM's. The vibration become apparent at 1800 then increase slowly to a peak at 3,000. There is no noticable increase from 3,000 to 4,500 RPM. You could feel the vibration in the steering wheel and very little in the gas pedal. I put a clear jar of water on the dash to see how violent it was. I was hardly noticeable even at the high RPM. I guess because it is at such a high frequency.

I think I need them to pull the inspection plate off and check to see which side the balance weight is on. You say it should be toward the front. This should be the starting point. Then they can check the torque converter. Not sure what they are to look for. Any idea where to go next??
Last edited by Raygta on August 18th, 2011, 1:45 am, edited 1 time in total.
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Joined: November 18th, 2004, 4:20 am

August 18th, 2011, 2:51 am #7

The item I am referring to is bolted on and probably weighs 10 pounds.

The other weights are welded on.
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Joined: January 1st, 2004, 2:15 am

August 18th, 2011, 3:51 am #8

The 6316 crankshaft dampner was a part of 428CJ engines and was located at the front snout just behind the harmonic balancer. They did not bolt anything on to the flywheel or flexplate.
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Joined: July 17th, 2011, 7:07 pm

August 18th, 2011, 4:17 am #9

The flex plate has a weight welded near the outer rim as shown in the link below. I saw the one that came out and it matched the photo on the left in http://www.428cobrajet.org/id-flexplate.html . I did not see what went in, but they assure me that it matched. ( They called me when they got a flex plate to install as I had told them the motor was a 428, and they asked if the motor was a CJ? I had left this detail out. The first flex plate they got did not match what was on the engine. When the got the correct one they said it matched.) There were no other plates, balance parts or bolt-on items.

I have run the engine in park to high RPM's. The vibration become apparent at 1800 then increase slowly to a peak at 3,000. There is no noticable increase from 3,000 to 4,500 RPM. You could feel the vibration in the steering wheel and very little in the gas pedal. I put a clear jar of water on the dash to see how violent it was. I was hardly noticeable even at the high RPM. I guess because it is at such a high frequency.

I think I need them to pull the inspection plate off and check to see which side the balance weight is on. You say it should be toward the front. This should be the starting point. Then they can check the torque converter. Not sure what they are to look for. Any idea where to go next??
From your description of the vibration, I would take a guess, and say they used a 390 flexplate.

I once had a 428CJ Cyclone that someone had replaced the 428 with a 390, and used the 428 flexplate on the 390.

Opposite situation from what maybe happened to yours... but very similar vibration. Same effective "unbalance" situation.

Verify the weight is there before chasing any other possible solution. Just because they <em>say</em> it matched, doesn't mean they know to look for the weight.

Looking at the picture in your link, it looks like the weight is on the REAR of the flexplate. A little harder to check with the trans in the car. Use a small dental mirror and flashlight.

I know I said I thought the weight was on the front.... but..... old guy memory here.... <img alt="wink.gif" src="/images/wink.gif" width="14" height="14">

 
Last edited by galaxiex on August 18th, 2011, 4:29 am, edited 1 time in total.
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Joined: November 18th, 2004, 4:20 am

August 18th, 2011, 9:34 am #10

The 6316 crankshaft dampner was a part of 428CJ engines and was located at the front snout just behind the harmonic balancer. They did not bolt anything on to the flywheel or flexplate.
I have seen them and bolted them onto the flexplates.
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