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6.5L Vibrations, any ideas what it can be?

Husker6.5 said:
Are (all) the flex plate to crank flange bolts torqued down, or even tight? That wobble could be due to loose/backing out bolts and at different RPMs the harmonic vibrations would get stronger/weaker as it wobbles. Or, it could be a bent flex plate, too.
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The bolts were torqued and blue loctite when I installed it. It has less than probably 1.5 hrs runtime due to the vibrations. It definitely has me wondering.. for less than 60 I can put in a new one, kicking myself for not installing a new one when I re-built it. Lessons learned, lets hope that replacing the flex plate resolves it. I'll update this when I replace it, another weekend under the truck.:dead:
 
Scott_2255 said:
Vid of Flex plate while cranking engine. It looks warped to me but I don't know what tolerance it can handle. At the present time with how it vibrates I'd say none :banghead:

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ok now let's just think a bit here! why do they call it a flex plate. now once it is bolted to the convertor. then it will flex with the trans. now if you would here a clicking sound then now, I would say bad flex plate. clicking would indicate cracks in the flex plate. but i am sure you would have seen cracks when you installed the flex plate. i am still leaning on the damper, like i said before you can buy bad new parts .
 
patgallen said:
ok now let's just think a bit here! why do they call it a flex plate. now once it is bolted to the convertor. then it will flex with the trans. now if you would here a clicking sound then now, I would say bad flex plate. clicking would indicate cracks in the flex plate. but i am sure you would have seen cracks when you installed the flex plate. i am still leaning on the damper, like i said before you can buy bad new parts .
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True- but I have seen many 6.5 running with nothing bolted to the flex plate and never saw one moving that bad that wasn’t ruined.

Yes the balancer could be a defective one. A related question would be the pulley in front of that balancer. I can’t imagine a billet pulley causing that much chaos, but it would be worth seeing crank over without a belt on it. Aluminum in shipping getting bent?
 
They call it a flex plate because it isn't a flywheel! And, it is NOT supposed to flex in operation, as it takes only rotational torque load and no thrust load, unlike a flywheel that takes both when running with the clutch engaged and doesn't flex because of its thickness.
 

Flywheels are typically found on vehicles equipped with manual transmissions, while flexplates are used in vehicles with automatic transmissions.​

Flexplates are much thinner than a Flywheel
AKMI' Product: AK-3973497 Cummins ISC ISL Flywheel
​

As well as the ring gear it uses to connect with the starter (depending on vehicle and engine size), and are much, much lighter.

This is due to the fluid coupling of the torque converter, which eliminates the grinding of a clutch.

The lighter, thinner metal frame has an ability to flex across its main axis – bending side to side (hence the name Flexplate) – taking up motion in the torque converter as the rotational speeds change.

The metal frame of the flexplate itself will have multiple machine-cut holes within the body of the plate.

One set of holes will look uniform and would be for mounting to the crankshaft.

The other holes are specific to the vehicle, torque converter set-up and potential weight balance of the flexplate.

No matter the car size, small economy size or heavy-duty truck.

If your vehicle has a manual transmission and you have to step on the clutch to shift gears, your vehicle has a flywheel as part of your transmission. If all you have to do is put the shifter in a drive (D) and step on the gas pedal, your vehicle has a flexplate.
 
The flex in a flex plate is to compensate crankshaft thrust and torque converter slide.

These are equal amounts. The ENTIRE exterior of the flexplate moves at a different fore/aft position to the ENTIRE center ring of the flex plate. It should not wobble as we see in the video. If that were to be continued to run that plate will crack- no two ways about it.

There is about a thousand videos on youtube you can watch of engines running showing the flex plate maintaining a true line.
 
patgallen said:

Flywheels are typically found on vehicles equipped with manual transmissions, while flexplates are used in vehicles with automatic transmissions.​

Flexplates are much thinner than a Flywheel
AKMI' Product: AK-3973497 Cummins ISC ISL Flywheel' Product: AK-3973497 Cummins ISC ISL Flywheel
​

As well as the ring gear it uses to connect with the starter (depending on vehicle and engine size), and are much, much lighter.

This is due to the fluid coupling of the torque converter, which eliminates the grinding of a clutch.

The lighter, thinner metal frame has an ability to flex across its main axis – bending side to side (hence the name Flexplate) – taking up motion in the torque converter as the rotational speeds change.

The metal frame of the flexplate itself will have multiple machine-cut holes within the body of the plate.

One set of holes will look uniform and would be for mounting to the crankshaft.

The other holes are specific to the vehicle, torque converter set-up and potential weight balance of the flexplate.

No matter the car size, small economy size or heavy-duty truck.

If your vehicle has a manual transmission and you have to step on the clutch to shift gears, your vehicle has a flywheel as part of your transmission. If all you have to do is put the shifter in a drive (D) and step on the gas pedal, your vehicle has a flexplate.
Click to expand...
Basically, I said the SAME thing in a lot fewer words and a lot smaller type size! I of course, and perhaps wrongly, assumed the readers of this thread were familiar with basic drivetrain principles and differences of an automatic vs. a manual transmission.

The flex plate in question in the video above has either a permanent bend in one of the "arms" or one of them is cracked all the way across to put that kind of wobble in it at just cranking speed.
 
oh, I agree with that it will crack soon, but as for the vibration if you blame the flex plate you may as well say transmission is also to blame it is connected to the flex plate. I am pretty sure the vibration is in the front of the engine. as Scott said he replaced the timing chain but did not replace the gears, also as i stated the damper could be bad, everyone can buy brand new broken parts. that's why I said I believe he should go to the junk yard and see if he can get an old pully and damper just to try and see if thing change first before tearing it down. it may solve all his troubles.
 
SORRY ASKED HIM ABOUT THE CHAIN AND GEARS HE SAID CHIAN ONLY. AND YES JUST THINK IF THE GUY AT THE FACTORY WHO SITS AT HIS STATION AND JUST NOT HAVING A GOOD DAY COULD BE THE CAUSE OF THE PART BEING BAD MIGHT BE THE ISSUE ALL I CLAIM IS THAT BEFORE HE TEARS IT ALL BACK DOWN IS TO TRY A FEW EASY THINGS TO NARROW IT DOWN. PLUS, THE COST HOW MUCH COULD IT COST YOU AT THE JUNK YARD FOR A STOCK DAMPER JUST TO TRY AND SEE IF THE VIBRATION CHANGES OR STOPS.
 
Oh, and chains don't use "gears", they use sprockets, gear drives like what Leroy sells use gears. And unless there's obvious signs of tooth wear on one or both sprockets, while highly recommended, sprocket replacement isn't mandatory. It is very, very unlikely the sprockets and chain are the source of the vibration. And lots of luck trying to find a rare 6.5 at a parts yard - let alone one with a newer "good" harmonic balancer that's not a Dorman or Chinesium P.O.S. that is of unknown integrity. That bent flex plate is about 98-99% likely to be the culprit of the RPM-dependent variable vibration - the same way a bent tire rim with a mounted tire on it will give a dynamic tire balancing machine a conniption fit when rotated at higher RPMs.

What assembly error would cause the Fluid Damper to be the culprit? Aside from their quality control being much, much better than the mass-produced elastomer/steel ring harmonic balancer of GM, let's address that for a moment. No viscous silicone fluid put inside of it? NOPE! It wouldn't pass the automated QC/QA and make it to the end of the assembly line, much less out the door. How about the computer controlled welding? Say for some reason there's excessive weld at one point that makes that spot 5g heavier. Guess what? Does NOT matter because the very design principle of the spinning viscous silicone fluid inside the ring SELF BALANCES the entire dampener assembly (as well as the rotating crank assembly when installed on the front of the crankshaft).
 
@Scott_2255 just out of curiosity, looking at your flex plate. can you physically see the counterweight that was spot welded on it from factory? there should be a weight on it with our engines being externally balanced. since it has some wobble to it when spinning I am almost wondering if it somehow got knocked off + warping the flex plate in the process when installing the engine.

Here is a pic of the flex plate for our trucks with the counterweight welded to the inside (side toward the engine block) @Will L. @Husker6.5 I just noticed there is some conflicting info on RA's website as to the # of teeth. for what ever reason 168 tooth lights a bulb in my memory but the delco one shows 142 and the other brand shows 139. I do know the outside diameter is 14.125 inches

1677526221033.png
 
Scott_2255 said:
The bolts were torqued and blue loctite when I installed it. It has less than probably 1.5 hrs runtime due to the vibrations. It definitely has me wondering.. for less than 60 I can put in a new one, kicking myself for not installing a new one when I re-built it. Lessons learned, lets hope that replacing the flex plate resolves it. I'll update this when I replace it, another weekend under the truck.:dead:
Click to expand...
I certainly hope too a new flexplate fixes your problem.I was going to buy a new scat crank for my 6.2 but I think I’ll hold off for now and see how things turn out with your engine.

I wasn’t aware the scat crank is lighter than the GM crankshaft.Good info everyone 👍
 
As mentioned in your original post, make sure your alignment dowel is lined up with your flexplate and that it was present on your Scat crankshaft when you added the flexplate. Obviously position matters as a counterweight in the wrong position would cause vibration if it was out 180 degrees specifically. Maybe someone could comment on counterweight position relative to torque converter and if possible to mount on "wrong side". If you could reverse the flexplate accidentally on the dowel, it could make a 180 difference to the counterweight. Can the dowel be 180 degrees out?

It would be interesting to know how and when they add the counterweights to the flexplate. I always assumed crankshafts are balanced by drilling holes in them. Is flexplate weights used to counteract small weight discrepancies in rods and pistons or is it for self balance?
 
So for someone reading this that may not understand the difference between external balancing and internal, internal balancing does all the balancing on the crankshaft so the flex plate or flywheel is zero balance or no weight added. This includes the harmonic balancer. Everything balancing wise is done internal to the engine hence the term. External balance depends on weight being added to the flywheel or flex plate and the harmonic balancer. When a balancing job is done to an externally balanced engine they normally don't touch the flex plate or harmonic balancer as weight was added to them in the manufacturing process. The crankshaft is drilled or weight added to make it balance.
 
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