Acpantera wrote:I really don't think the boots are meant to seal or protect the components in the rear arm, It's more for the looks and to fill the pivot gap. If you inspect the rear drive from the bottom side you will see a 3/4" hole in the bottom of the final drive cavity to drain any fluid that may get in there(hardly an air tight seal). The u-joints are the automotive type and quite weather proof, just like many cars and trucks use. Each trunnion of the joint has its own seal to keep lube in and contaminants out. The grease in any component has a service life, it all depends on temps, speeds,loads, integrity of the assembly and how well it was put together. I'm sure that the lack of, or improper maintenance is not the fault. After 75,000 miles of hard use I would chalk this up to fate.
Look a CLOSE look at your rear drive again. Stick your finger in. Wiggle it around. The 3/4" hole does NOT go into the universal joint area. When the rear drive is swung down to change the oil, there is no corresponding hole on the interior - that is sealed by the boot. The rubber boot makes it a sealed space.
If you look at the photo:
You'll see the inner ring part of the casting where the rubber boot seals to.. the round part with grease on it (which is correct - the grease seals the rubber up.) The rubber boot has a corresponding round protrusion that seals to this ring (when installed correctly.)
The spline and universal are actually inside the boot area (clever design since the axis of the universals are in line with the pivot points, meaning there is virtually no change in length between the transmission and rear drive when the suspension moves up and down, so much less likely to have spline wear as earlier BMW drives were prone to.)
The rubber boots
are there to seal the swingarm up tight - that's why the plastic retainers are installed inside them. The difference with the U joints in this application and a car - if water gets into the swingarm area due to a poorly installed boot, and the bottom of the accordian boot is still sealed (it usually is - the top is the part that doesn't seal too easily) - then the water stays in the swingarm, and starts rusting the U joint. Once the trunnions are rusted, the seals rather quickly go, and next are the bearings. If you look at Joes photos you can see corrosion on the trunnions. The only way that can happen is if the seals go.
On a car the universal joints are not in a semi-sealed enclosed space that can retain water.. so the rust doesn't get a chance to start.
To make sure it seals up correctly - I always seat the boot first on the rear-drive, and make certain the rubber is around the ring and the plastic expander/retainer is in place. The joint to the swingarm itself is easier to get correct when the rear drive is then swung up into place.
Did Joe give it hard use? Yup.. but in this case - some sloppy shop work (Joe mentioned he didn't do the last rear-drive oil change) was likely the cause of the failure. I'm sure if it had been rusting that badly when Joe changed the oil himself prior to the shop doing it that he'd have noticed it.
