A friend of mine had some problems recently with the drive shaft (carden) on his R25/3. The u-joint was completely shot from neglect, and I think also from poor installation from somebody else, the u-joint had already been replaced at some time before.

I did this job quite a while ago on my r25/3 so doing another prompted me to write something up on it.

The u-joint is a quite difficult to get out because, as with most u-joints on motorcars there are no clips holding the cups into the yokes of the drive shafts. On the r25/3 they are simply centered, trued then spot welded into position. It’s those spot welds that are difficult to remove without taking too much of the metal away from the shaft yokes. Luckily we have the dremel so those welds can be carefully ground away to release the caps. It takes a while of careful grinding with a small stone so that there are no burs on the edge so that the cups can be pushed out from the opposite side.

To the left are all the bits that go into making up a new u-joint. Of course the joint comes fully assembled, however, since it can be such a fiddly job you may have to rebuild the joint itself. This is especially true when first positioning the cross of the u-j in the yokes and sliding the cups with roller bearings through the locating holes over the cross bearing ends. If one or two of the rollers move, then it’s a case of pulling the cup out and starting again. If you’re at this stage and the u-j is completely apart use a nice and sticky bearing grease to carefully replace the rollers.

When the new u-j is finally installed the most important part is making sure that the shaft is running true. It may be possible to do that while the shaft is static but I think it would be almost impossible considering all of the components are old except for the new u-j itself. So we need to set the shaft up it a lathe or some other device so that we can spin the shaft and true it. It’s very important that the shaft is running true to avoid an early failure of the u-j. If they are set up correctly they will last a long time

To the left is a picture of the gearbox flange end of the carden shaft held by a floating or running center in the tailstock.

This is the rear drive end of the shaft held in the lathe chuck. It’s not necessary to clamp the chuck very tightly because we are just holding it in position to spin it. We also want to avoid putting any nicks or cuts into the surface as this is our seal surface and must be in perfect condition to avoid oil leaks. Suffice to say that it should also be clamped hard enough to hold it securely while it is turning under power.

Something that isn’t showing in this picture is a means of measuring the run out. I could have set up a dial gauge to measure the run out, but you can also set up a simple pointer which is what I did, you can get very accurate results with a fixed pointer and a good eye. Either way is good enough.

Once your happy with your truing of the shaft, it must be spot welded. If the shaft is running true not a lot of weld is required to keep the u-joint firmly in position. Since I don’t have a real spot welder I used a simple stick arc welder with a thin stick to place a tack on the cup and yokes. Be careful not to disturb the shaft while applying the weld, also be careful not to get any weld splatter on the lathe beds.