This problem may be a little bit misleading because I’ve got a hybrid of parts on my 1974 XJ12 with a 6 liter and 4L80 swap.

The majority of the driveshaft is the stock one from the 1995 XJ12 donor car that gave up its engine and tranny (and driveshaft). The only difference is that I reused the rear differential 4 bolt flange from the original ‘74 differential instead of the rubber guibo on the ‘95 car.

And so I had a local drivetrain shop make up the driveshaft to my specs which included a new aftermarket center support bearing.

I’ve been trying to solve a driveline vibration on acceleration since the engine swap and was able to substantially resolve* it but there still seemed to be a vibration upon hard acceleration for a few hundred RPM.

*I took some forum advice and moved the driveshaft over towards the passenger side and that helped. I then elongated the holes to allow the center support bearing to slide even further to the passenger side and that helped even more. But there was still something there…

I decided to try a new center support bearing and when I took out the “new” one I’d installed with the drivetrain swap roughly 18 months ago, that had maybe 500 miles on it, I noticed it was deformed. The bearing was not centered in the housing (see pics) while the replacement was.

Now, I have no idea if the aftermarket bearing I originally used was deformed from the beginning or if it deformed from sitting static for weeks and months at a time while I worked on other projects.

My experience with aftermarket parts tells me they suck but I’m also magnanimous about the fact that my set up is so unique that I can’t automatically blame a failure* on the part.

* is this a failure?? I’ve got to assume that the bearing should be centered but what do I know?!?

I’d like to know what y'all think. Bearing below has 500 miles and was installed 18 months ago.

 

Andy,
We have similar setups in that our engines, transmissions, and drive shafts are not necessarily common to each other or the car. I had a custom driveshaft built and balanced and felt the vibrations when I put the center bearing in the original position along with the stock 5/8" spacer. I made 1/4" and 3/8" test spacers and moved the bearing position back and forth.....results were good, better, and not perfect. I am not using any spacer now and can't tell the difference between bearing far right or far left, both are completely acceptable (to me). My suspension has poly bushes and that makes our roads telegraph a lot of "texture".

If you feel good about the balance job on the driveshaft, make a close inspection of drive-line i.e:
- U-bolts tight and cups seated; pinion nut properly tight
- Check the fit of the front sleeve/yoke that slides into the transmission, if the driveshaft is too short there won't be enough engagement on the trans output splines to keep the shaft running true. Conversely, if the sleeve is bottoming against the output shaft, the driveshaft is too long and may be binding and prevented from running true. I personally like 1/2"-5/8" of room for any movement with a stationary differential like ours. On that note, if your differential cage is mounted as stock and you are really twisting the tail on that 6L from a stand-still, the cage might be moving enough to force the driveshaft forward into the transmission, even overcoming the rubber suspension in the center bearing. If you think something like that might be a possibility, anchor the cage or give a little more room between trans output shaft and driveshaft sleeve.
- Make sure the machine shop "timed" the universal joints when they built your driveshaft. Take a look online to see proper arrangement.
- Make sure the motor mounts are tight. On spirited acceleration the motor/trans want to move around and if the trans mount isn't anchored it can throw the alignment out slightly.
- You can research online and find a number of "driveline" sites with how-tos and what-fors on driveshaft angles and myriad bits of other information. I layed under my car for a full day with a bubble level and home-made stuff, trying to make sense of it. Came to the conclusion that I would have to rebuild the whole car to get the numbers they indicated. I returned to the "trial and error" method.

As far as your existing bearing not being centered, if you can move the bearing around within the rubber mount without a great deal of effort, yet still feel that the bearing is being held securely, without rattling around, I think it's OK.

Dave

 

Thanks for those insights, I'm gonna read and reread. But one thing you said really stood out to me:
You can research online and find a number of "driveline" sites with how-tos and what-fors on driveshaft angles and myriad bits of other information. I layed under my car for a full day with a bubble level and home-made stuff, trying to make sense of it. Came to the conclusion that I would have to rebuild the whole car to get the numbers they indicated. I returned to the "trial and error" method.

I had the exact same experience, after reading everything simply gave up and started trial and erroring and that got me closer than I'd ever been.

 

The centre portion of the centre bearing is rubber mounted and can move a fair bit. If you have vibration I would suspect that the U joints are misaligned.

There is an SAE handbook on designing driveshafts, and I actually got it on interlibrary loan last year and read it - all 344 pages. While very informative, I came to the conclusion that for my application I couldn't get the correct angles with U joints, so I will use CV joints and not worry about angles. The angles must be small, ideally no more than 3 degrees in total, and a variance of no more than ½° across the joint. As an example, if the angle between the transmission output shaft centre line and the first part of the drivelshaft is 1°, then the angle across the centre joint and at the diff should also be 1°± ½°.

You need to do this in 2 planes too, looking from the top down on the car and from the side. So you might need to adjust the centre bearing side to side or shim it up and down.

The fact that the vibration changes when you move the centre bearing says the shafts are out of alignment. If you have the X300 shop manual there is a section on how to align the shafts.