put in 8 3/4, now have vibration

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screeminDemon

screeminDemon

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any suggestions on what might be the most likely culprit?

My car is 71 demon with 360 and 4spd OD tranny. I just put in 8 3/4 used carrier 741. Its 3.91 gear with suregrip.

Suregrip works good, noise is minimal, however it starts a very fine vibration close to 60mph then gets steadily worse as speed goes up from there.

I got new big bolt pattern axles with green bearings from Dr. Diff and everything installed good.

Note: the used chunk I got showed some corrosion on several teeth of the big gear but I figured it wouldn't affect anything...
 
SGBARRACUDA said:
or not centered in the rear end yoke.
Click to expand...

interesting... thanks for your input. I had the driveshaft shortened and balanced and they did some other work on it... total came to $200 bucks and it was painted and looked like new. They had my same u-joints and tongue on it... said they were ok.

I was wondering if I ought to replace the rear u-joint anyways... they're not expensive. But what do you mean by "not centered in the rear end yoke"?
 
Disconnect the driveshaft from the pinion flange, spin it 180º, and reinstall it. Most times this will correct the vibration you describe.
If it does correct the problem, mark the pinion & driveshaft so if you ever need to remove it, you can reinstall it the correct way.

George
 
The two cups that fit in the yoke are suppose to have C clips on them that center the driveshaft u-joint in the middle of the yoke, if they are not there it is impossible to install the joint in the center of the yoke. Make sure they are there. You don't have to replace the joint as long as the cups rotate smooth and there is no play.
 
great input guys, I appreciate it! I'm gonna work on it this evenin then I will update this post
 
SGBARRACUDA said:
The two cups that fit in the yoke are suppose to have C clips on them that center the driveshaft u-joint in the middle of the yoke, if they are not there it is impossible to install the joint in the center of the yoke. Make sure they are there. You don't have to replace the joint as long as the cups rotate smooth and there is no play.
Click to expand...
This is EXACTLY what happened to me, once I put the "C" clips on and reinstalled my drive shaft no more noise for me, hope you find your issue.
 
Don't ever forget to install C clips. I Had a bad experience once. U joint slid all the way to one side of the rear yoke while doing 80k, and it gave my 360 a death shake.
 
Another possibility is the pinion angle. If this is not correct, the u-joints will bind and the vibration will get worse with speed. Easy fix tough, with wedges under the spring perches.

Buy a cheap angle gauge from Horror Freight to check this.

Good luck!
 
Update: Under my car last night messing with the u-joint (the C-clips were on) and discovered it has some side to side play of about 1//32" or 1/64" because apparently the c-clips are not fitting snug into the pinion/yoke where it seats.

I slid it tight to one side then cranked down the little hold down bolts (not too tight because I've broken one of those before). WHen I was driving my car to work on my 10 mile freeway drive this morning I was amazed at how smooth it was.

Then suddenly it started vibrating again, seemingly worse then before. I assume the u-joint got loose and started moving (vibrating) in the yoke/pinion.

I dont know why the u-joint doesnt fit tight when seated. The c-clips should be scraping the edges when seating into the yoke, but such is not the case. I'm goint to buy a new u-joint and hope it fits snug...
 
Sounds like you may have a 7260 small joint on your driveshaft in a large joint 7290 pinion. If so, just purchase a conversion joint that has a small end for your driveshaft and a big end for your pinion. Also make sure that the straps you are using are for the right size u-joint.
 
Update: I put in a new u joint with part#6301 which seemed to be a better fit. AutoZone guys weren't real sure about which u-joint to use since I've swapped out my entire drivetrain (I dont even know what year make and model my rear end came out of).

I still have the vibration, seems the same. I think I must have been hallucinating yesterday when I thought it was better for a minute.

Can anybody tell me the most common part number for u-joints used in 8 3/4 rear end? I'm still concerned because the one I put in has a slight amount of play (very slight, not measurable) and I remember in the past that the c-clips were usually a very tight fit into the yoke. Plus - the cups of the u-joint are recessed into the yoke (like the u-joint is too short?) not extended out.

Thanks for your help and any other suggestions!
 
68gtxman said:
Another possibility is the pinion angle. If this is not correct, the u-joints will bind and the vibration will get worse with speed. Easy fix tough, with wedges under the spring perches.

Buy a cheap angle gauge from Horror Freight to check this.

Good luck!
Click to expand...

Horror Freight, LOL I like that place! But anyways I didnt know anything about the pinion angle, do you know what the angle should be?

Here is a couple pics of front and rear ujoints: PS - who thnks the problem might be caused from my driveshaft a little too short?
 

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Wow, that slip yoke should be in the trans more for sure. How did you measure the length for the shop? I would not drive it until the problem is corrected, you could damage the tailshaft.

200 is quite high for a shorten and balance IMO, I had mine done for ninety and they installed new joints, painted and balance to perfection.
 
That U joint looks like a conversion 7260/7290
6081.jpg

and the shaft is sadly too short now. There should be about 3/4-1" of shaft showing at normal ride height. You got 3 options: new longer shaft, SS springs that move the axle forward a few inches or a DANA 60 rear that is longer in the snout - $$$.

The U joint are both internal circlip and should self center with no play. Tap in cap and put Circlip on, tap on the other and itll seat the opposite one with just enough to get the 2nd circlip on. The Diff side is easier, the clipped caps just seat in the yoke and the straps bolt on.
 
For most daily street use, the plane of the trans output shaft and the rear pinion shaft need to be parallel within +- 1 degree (2 degrees total deviation). The angle of offset between the driveshaft and u-joints needs to be between 1 and 5 degrees. (quoted from Hot Rod magazine article.)
 
How much of the slip is in the trans ? before we just go running out and get a new shaft and then find out that slip is 9" long and you didn't need a new shaft.

You said the vibe started after going to the new gear, so you already had this entire driveline in the car with no issues.

So what was the gear ratio in the old 8.75 ?

Seems to me you may have made a big enough jump that you just need your shaft balanced
 
Also, I am not certain, but it looks like your yoke is made for the big joint. Take the joint out from the driveshaft and try fitting the larger end into the yoke. If it fits well, you should use a standard 7290 (big) joint. I think that is all it could be as the 1350 stuff uses U-bolts instead of straps.
 
Supershafts said:
How much of the slip is in the trans ? before we just go running out and get a new shaft and then find out that slip is 9" long and you didn't need a new shaft.

You said the vibe started after going to the new gear, so you already had this entire driveline in the car with no issues.

So what was the gear ratio in the old 8.75 ?

Seems to me you may have made a big enough jump that you just need your shaft balanced
Click to expand...

I took the shaft out last night, then re-installed it. I measured/estimated that the tongue/slip engages about 2.5" of tranny spline (tailshaft?).

I know it should be more like 4.5" however I'm still not conviced this is whats causing my vibration. Maybe... but maybe not, I dont know.

The ENTIRE drivetrain was not replaced, just the driveshaft and rear end. I was already running my new 360 and a-833 without vibration for about a year... on the original 7 1/4 rear end... probly 2.94 gear.

Thanks for you help! PS - I already had the shaft balanced (and they replaced parts of it which cost me close to $200). The shop claims they shortened it according to my request (2") and tells me I'm to blame for giving wrong instructions. I wish I had given written instructions and dimensions instead of just verbal... cuz I'm convinced they screwed up and shortened it too much. Is it possible to get more length out of it? Change the slip or something?
 
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