HELP!!!!Put in 8 3/4 rear and it vibrates real bad

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loganscuda

loganscuda

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I just installed an 8 3/4 with new bearing and 355 gears and at about 30 mph it shakes really bad what the heck is going on. it is real bad.
 
first off its not tires or rims is it? when raised on stands does it still do it?

was gear mesh checked? does there sound like metal on metal?
 
loganscuda said:
I just installed an 8 3/4 with new bearing and 355 gears and at about 30 mph it shakes really bad what the heck is going on. it is real bad.
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Pretty vague info.

Did you rebuild the unit? How about pinion angle? What car? The more info you provide the better for someone to hopefully narrow things down for ya.
 
I did not rebuild it just new bearings. green bearing. It is a B body 8 3/4 out of 66 charger. In 67 barracuda. Had an 8 1/4 in it used same wheels and tires with no problems. It has a 741 case. Up front is 5.7 hemi with 545rfe trans. I have clearance on tires all around. I will put it on stands and try it.
 
Is it a suregrip or peg leg. Reason I ask is if it's a suregrip , did you remove the thrust buttons/block? Did the bearings seat properly? What about pinion angle? Did you remove the perches and weld them on narrower.

And lastly what about your drive shaft? Shortened and balanced??
 
Loosen all the lug nuts, make sure the wheel seats flat against the hub and re-torque.

Remove the straps holding the drive line to the rear end, flip the drive line 180 in the yoke, re-install and re-torque.

Both of these cost zero money and I have seen them fix similar problems.
 
What about your u-joints are they new or used? did you loose any needle bearings. is it properly centered? Off balance?

You can get vibration from the driveshaft also.

Another thing to check before spending any more money.
 
krazykuda said:
What about your u-joints are they new or used? did you loose any needle bearings. is it properly centered? Off balance?

You can get vibration from the driveshaft also.

Another thing to check before spending any more money.
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Totally different vibration, but try explaining it to someone over the internet.

How much wobble at the wheel might be nice to know.
 
Ok wheel looks like it warped when turning, but the wheel is not swapped wheels still does it. But It looks like it is the drive shaft. Son put driveshaft on and where the u joint goes on is 4 1/4 inch which is 1 inch bigger than my u joint in width. What kind of car would this center section come off of. I have another one that is the same as my 8 1/4 was.
 
loganscuda said:
Ok wheel looks like it warped when turning, but the wheel is not swapped wheels still does it. But It looks like it is the drive shaft. Son put driveshaft on and where the u joint goes on is 4 1/4 inch which is 1 inch bigger than my u joint in width. What kind of car would this center section come off of. I have another one that is the same as my 8 1/4 was.
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What? OMG!
 

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loganscuda said:
Ok wheel looks like it warped when turning, but the wheel is not swapped wheels still does it. But It looks like it is the drive shaft. Son put driveshaft on and where the u joint goes on is 4 1/4 inch which is 1 inch bigger than my u joint in width. What kind of car would this center section come off of.
It is a B body 8 3/4 out of 66 charger.
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I have another one that is the same as my 8 1/4 was.
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That's why I was curious about the drive shaft. If you replaced a 7 1/4 with the 8 3/4 the drive shaft, for one thing, it would be too long. It might be possible that something got forced out of shape when you put the weight of the car on it and drove it.. I would also look closely at the tailshaft on the trans to be sure it's not damaged.

There are combination U joints that will fit your driveshaft AND fit the larger yoke on the 8 3/4.
 
roadrunnerh said:
Are you saying you went from 7260 (small) yoke on rear to 7290 (larger) yoke on driveshaft?
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I thought he was saying they put the small joint that was still on the driveline in the large yoke without noticing.
 
Bad Sport said:
That's why I was curious about the drive shaft. If you replaced a 7 1/4 with the 8 3/4 the drive shaft, for one thing, it would be too long. It might be possible that something got forced out of shape when you put the weight of the car on it and drove it.. I would also look closely at the tailshaft on the trans to be sure it's not damaged.

There are combination U joints that will fit your driveshaft AND fit the larger yoke on the 8 3/4.
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That would def cause a vibration (More like a nasty shudder)
Trans mount, and engine mount checks would be in order also if the driveline was long and got put in and driven.

I was under possibly the wrong impression (that you couldn't get the longer driveline in because of the length difference.) ???
 
If the rear is the popular Mopar "large" joint (inside clips) and the shaft is the popular small joint, there's a conversion joint for that.
 
I am grateful for the advice. thanks and you are all welcome for a big dose of confusion from me.
 
Better yet ....take a picture of where the driveshaft u-joint connects to the rear and post it on here so we can see what it looks like
 
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