Hey C20 Owners

[tdangle]

Do you have vibration issues with your stock setup? Mainly those with leaf spring rear suspensions

I have vibrations which I've narrowed down to the driveline. It is speed dependent, not tire/wheel/brake drum related. I've gone thru everything shimming the pinion angle, shimming carrier bearing and trans mount. Up down everything I've tried only seems to move the vibration around but never eliminating it.

I've had the 2 piece d/s rebuilt and balanced twice, new motor/trans mounts. duplicate the issue with rear on jack stands with no brake drums/tires/wheels.

I can't believe GM built this truck to vibrate. it is totally stock suspension.

[jocko]

My C20 350/TH350 D60 with leafs didn't vibrate.
Since you hear/feel the vibe on jack stands with no wheel/tire, it leads me to believe that the issue is in the angle between your trans (which appears to be the only non-stock item) and the carrier bearing. The relationship between the carrier bearing and rear end should not have changed with the trans swap, so shimming the pinion may have only added more variables to your equation (or made things worse) - same for the carrier bearing shims. Can you describe the trans mount? Did you buy it with the 700R4 in it or did you install? If the latter - was there vibe before the swap? Have you checked/replaced u-joints?

[tdangle]

Really puzzled on this one. I don't know if it had the vibration with the stock TH350 as I really didn't drive it much especially on the fwy. Honestly I don't remember anything but tire balance issues back then. Then I installed a 2004R and again I'm not sure when the vibration started as there were so many other issues at that time. Now the 7004R. Wish I had been more aware of possible angle changes before swapping trans. It is a new stock mount, I was told it is the same mount for TH350, 400 2004r and 7004r. When it was a TH350, it had a .4" spacer between the top of the mount and the trans. I kept it in place when changing trans. Only thing that changed was the position of the trans cross member. I would love to know the distance from the mounting surface to center of the output shaft on a TH350 to compare to the 7004R and angle of the trans with a TH350.

[Richard]

You mentioned stock setup and leaf springs. If they are stock the spring and shackle bushings are 50 year old rubber. Maybe a take look if you have not.

[RichardJ]

Dana Spicer Driveline Installation.

https://www.waterousco.com/media/wys...311-1-DSSP.pdf

This may be of help.

[tdangle]

Originally Posted by Richard

You mentioned stock setup and leaf springs. If they are stock the spring and shackle bushings are 50 year old rubber. Maybe a take look if you have not.

Replace them. no change, but they were frozen in place.

[Mickeymotormount]

Be sure the driveshaft center support rubber is soft and pliable. I had a squarebody with driveline vibrations. Drove me crazy until I looked closely at the center support rubber surrounding the bearing. Not torn but rock hard. It couldn't absorb any small vibrations in that condition. I changed it out for a new one and it solved the problem.
Make sure the driveshaft phasing is right as well.

Good luck.

[special-K]

I owned a C/20 I traveled all over the country in, no vibration. It had leaf spring rear and was stock original equipment. Tires on the rear were 33/12.50-16.5s and the truck ran smooth.

I'm wondering if the transmission isn't the culprit.

I had 4wd with divorced transfer case that I managed to twist the intermediate shaft in two on. I had a machine shop repair it. It vibrated really badly, shook the whole truck. I went back and asked if they balanced it and they said, "No, we align them. Balancing is not necessary". I grabbed it up and before walking away I held it up and said, "I could prove you wrong". I ordered a new driveshaft from the dealership. Back to smooth operation

[tdangle]

Originally Posted by Mickeymotormount

Be sure the driveshaft center support rubber is soft and pliable. I had a squarebody with driveline vibrations. Drove me crazy until I looked closely at the center support rubber surrounding the bearing. Not torn but rock hard. It couldn't absorb any small vibrations in that condition. I changed it out for a new one and it solved the problem.
Make sure the driveshaft phasing is right as well.

Good luck.

Had entire driveshaft rebuilt (including Center Bearing) and balanced when I had the 2004r installed. Still had vibrations. Later when I swapped trans out to a 700r4 (way better trans) I had the driveshaft shortened and rebalanced. Also they marked it for phasing.

[jeffahart]

Originally Posted by tdangle

Had entire driveshaft rebuilt (including Center Bearing) and balanced when I had the 2004r installed. Still had vibrations. Later when I swapped trans out to a 700r4 (way better trans) I had the driveshaft shortened and rebalanced. Also they marked it for phasing.

Here's something you might try. Loosen the bolts on the u-joints and knock them loose. Then snug them only... don't cinch them down. Try another run on the jacks. If the vibration goes away it could be a couple things. One, overtightening the joints. OR, possibly a disfigurment where the joint seats causing it to bind once tightened. An over tightened joint can bind and cause vibrations. Empire driveline sends a giant pink card with their shafts when they ship them... the card is a whole lecture on not over tightening u-joints.

Easy test anyhow. Good luck,

j

[tdangle]

Originally Posted by jeffahart

Here's something you might try. Loosen the bolts on the u-joints and knock them loose. Then snug them only... don't cinch them down. Try another run on the jacks. If the vibration goes away it could be a couple things. One, overtightening the joints. OR, possibly a disfigurment where the joint seats causing it to bind once tightened. An over tightened joint can bind and cause vibrations. Empire driveline sends a giant pink card with their shafts when they ship them... the card is a whole lecture on not over tightening u-joints.

Easy test anyhow. Good luck,

j

I'll try it and report on the outcome. I'm open to anything at this point.

[notsolo]

If you have changed out trans and drive shaft it does not seem like it would be u joints. Sounds like you have swapped out everything except the rear axle itself?

[72 paul]

Had problem as yours,was crankshaft in mine.Vibrated at different rpms after changing tourqe converter, rotations of converter,balancer change, flexplate x,2 other rebuilt 700r4s.Drove me nuts.Turns out machine shop used the same crank that was making it go crazy.Got lucky by measuring back of crank where mates up to tourqe converter.Yank engine out and owner of shop came over and measured said he never ran in to anything like that before.He made it rite though.

[DANTIP]

I had a vibration in a '72 K20 that would come on around 50 mph and stay until 65 or so. I had the driveshaft balanced and indexed. The rear differential was a GM corporate. I later had a Dana 60 modified and installed in the truck and the vibration was gone.

[tdangle]

Originally Posted by 72 paul

Had problem as yours,was crankshaft in mine.Vibrated at different rpms after changing tourqe converter, rotations of converter,balancer change, flexplate x,2 other rebuilt 700r4s.Drove me nuts.Turns out machine shop used the same crank that was making it go crazy.Got lucky by measuring back of crank where mates up to tourqe converter.Yank engine out and owner of shop came over and measured said he never ran in to anything like that before.He made it rite though.

Your vibration would have been RPM dependant then, correct? Mine is speed related. Same in 3rd or OD at ~40-50 mph. Or around 2K driveshaft speed.

[tdangle]

Originally Posted by DANTIP

I had a vibration in a '72 K20 that would come on around 50 mph and stay until 65 or so. I had the driveshaft balanced and indexed. The rear differential was a GM corporate. I later had a Dana 60 modified and installed in the truck and the vibration was gone.

I'm actually starting to suspect the Dana 60 as the problem. Just can't figure out how to prove it.

[72 paul]

Was a real pain to figure out.I could unhook converter run it up and no shake,hook back run it vibrate.Only did under a load. Everything rebuilt or new in running gear,posi rear.fresh driveline.The works.Wanted all roller engine built with forged crank and mild cam,just a reliable deal.Some wierd crank was put in that machine guy said never ran across in all his time 35 40 years of doin.Changed it out and no more problem.Good luck...I dont go to that shop no more..

[tdangle]

I wanted to update this thread. I pulled the back half of the drive shaft and rotated it 180 degrees relative to the front half from what the drive shop had marked. Probably close to 90% of the vibration is gone. Who knows why this worked, honestly does not make sense to me.

[LockDoc]

Originally Posted by tdangle

I wanted to update this thread. I pulled the back half of the drive shaft and rotated it 180 degrees relative to the front half from what the drive shop had marked. Probably close to 90% of the vibration is gone. Who knows why this worked, honestly does not make sense to me.

Yes, you would have thought that the balance would have been spot on. Maybe it was, but there may be other factors that are at play here like angle, phasing, or binding. Glad it got better anyway.....

LockDoc

[Bigdav160]

Originally Posted by tdangle

Who knows why this worked, honestly does not make sense to me.

Stacked tolerances.

Each piece is individually balance but if installed with all the heavier part on the same side then it can exceed the specification.

Imagine if both driveshaft pieces meet the maximum runout specification of .001". If install incorrectly (stacked tolerance), together they would have .002" and exceed the maximum.

[72 paul]

Glad to here you found problem..

[PbFut]

Tdangle Thank you for the update. Good info in this tread.