Hey guys, just replaced u joints on rear drive shaft and didn't mark front and back. Is there a way to tell and does it matter? I know ideally it should go back exactly how it came out, but I messed up. Is this major???

 

Not at all. What's more important is that the 2 pieces of the shaft (slip yoke section, and main shaft section) remained in the same orientation to one another during the job.

 

well that's a relief.
Well there's a 50% I put it back correctly! Hahahah.
there are two dots of metal on the shaft which I'm guessing is balancing?

 

Yes, that's what they are. The 2 sections of the shaft were balanced together when manufactured. So keeping them in the correct orientation matters. If you change their orientation, it can cause vibrations. Its not the end of the world, but a hassle.

 

Do you know which dot goes at the front and which at the back? If it is wrong I'd rather change it. At the moment the larger dot is at the back (axle side) and the smaller one in the front

 

You'd probably have to post some details about your vehicle and a couple good pictures to make sure we are talking about the same thing.

 

Jeep Cherokee XJ 2000 sport 4.0 Stock standard, no lift



If if you look closely either end of the shaft there are the balancing dots. There is a small and a large dot, do you know which side is supposed to be on the slip yoke side?

 

Now I see what you mean. No, there is no easy answer there. They would have put the balancing weights however they needed to when it was manufactured. I just cross checked against my own pictures from when I rebuilt mine, and there is a balance weight at each end of the main section and they are the same size weights. If you lost the orientation, you are probably just going to have to put it together as best you can and see if anything comes of it. If you get rumble back there, you'll either have to try a few combinations until you get it right, or take it someplace that can put it back together in the correct orientation and check it on a balancing machine.

 

To make matters more confusing, it is not unusual to get a little bit of rumble from the new u-joints for the first 100 miles or so as they settle in to their angles. It may be difficult to differentiate between something being unbalanced during that time.

Like I said, a hassle, but not the end of the world (and probably drive-able as long as you don't get severe vibrations).

 

Ok so they've not all got the same size factory weights? You see I saw a friend of mines drive shaft from an XJ today loose on the floor, and noticed that he also had a small and large balancing weight, but they were on opposite sides when compared to mine, which is when the penny dropped.
You see I've already installed, but have the valve cover off for another job, so can't test drive just yet.

 

You'll just have to roll with the punches one you get to test drive it. Its a solvable problem, so don't sweat it.

 

cheers man, ill do that.
Looked up on some other forums and it seems that it really makes no difference. With some vehicles that has a slip yoke in the drive shaft, with joints at the ends, people have recommended swapping them for various reasons, so I'm sure this should be fine.

My new U joints did feel quite tight...you said there may be some grinding to begin with? Should I hold off the paranoia for the first couple 100 miles?

 

Doing one end at a time seems a simple solution.

 

Ok so took it for a lengthy test drive today, all seems well. The vibration I was getting in the seat when under load is now gone, so defo was the u joints.
the driveshaft seems to be balanced, so either I put it on correctly or it doesn't matter.
There is an occasional drone from the driveshaft, but has been getting less and less...I'm guessing it's joints just settling.
All good for now I think, hitting a mild trail tomorrow so we shall see