Cups should be tight, pressed in.

What brand u joint did you use? China?

Measure the driveshaft/ u joint opening to see if it's in spec.

 

I'll measure the DS.

The cups slide in on the driveshaft side, snug fit on the yoke and diff mount (steel?)

This s is the ujoint I have. I didn't see country of mfg.

 

I quit using aluminum drive shafts because of this issue.

 

What’s the issue? I mean a u-joint is a u-joint no matter if it’s in an aluminum or steel driveshaft isn’t it? Just trying to understand what the issue would be in an aluminum DS and not trying to be offensive in how the question is coming off in text.

I ask too because my 94 Cobra has an original Ford Motorsport aluminum DS and hasn’t had any issues in the 18 years it’s been on the car.

 

I just went through this. Aluminum is softer than the steel u joint caps. If a cap spins, it easily wears into the aluminum yoke. In my case, the pinion angle was way off and caused binding in the u joint. It's very common is 4 wheeling. Extreme articulation causes steep u joint angles which cause the caps to spin in the yokes. When left unchecked, the caps wear into the yoke and cause them to elongate. It happens with steel and aluminum shafts.

Here's another thing to consider... I've now had two Ford Motorsports aluminum shafts that caused vibrations right out of the box. I took them to a drive line shop and they were found to be out of balance. My current shaft now has a weight on each end of the shaft. Problem solved.

 

I’ve had my ford Motorsports aluminum ds installed at mustang magic in 1998. $150 lol

just rebalanced 3 years ago because I added a 31 spline yoke. Not needed but I’ve never had it done. It was fine

 

fasterthangas is right on with what is happening.

I tried spinning the caps on the diff end and I could spin them by hand in the aluminum.

When I replaced the rear joint, the caps were snug. I remember them needing a light tap to get them to install. I wouldn’t say they were really tight like the yoke though.

Guess I’ll throw my old stock driveshaft back in. Probably see about getting it balanced and new joints in that one before it goes back in.

 

Why does something like this happen?

 

Read my initial reply. Those caps are relying on friction to hold them in place. Basically a press fit. Just think about the forces placed on those caps when there's drive line vibes and extreme angles. The u joint can only rotate so far which results in a cap or caps spinning. Something has to rotate until complete bind occurs. Once a cap spins, it will always spin. It's easy to see if you look at your joints. If a cap or it's bore looks shiny, it has spun.
Then there's the fact that not all joints are created alike. Cap diameters very between brands. I always buy Spicer joints but I first measure the cap bore in the yoke and compare it to the diameter of the caps on the new joint.

One other thing to note...
If a cap is driven into the yoke at an angle; it will gouge the aluminum yoke. This results in less material present to grip the cap. Joints should always be pressed in.

 

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