Limited Slip Diff Notes

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Limited Slip Diff Notes

Slight project from this weekend and I thought I would share. A good friend was going to replace the side bearings in his diff on his C1 and I thought it would be a good opportunity to refresh my mind about how these diffs are constructed.

After removing the diff from the car, I am sure I do not need to go into detail on this since every e21 owner knows how this is done. We drained the diff and removed the half shafts, the finned cover and the side seals. After this the limited slip clutchpack just falls out. To remove the bearings without using a press we heated the bearing itself and used a pry bar to remove them from the case. Since the case was clear and curiosity go the best of us we removed the side cover and looked at the internals of the LS unit. The insides are washer, wavywasher, lockwasher, clutch, inner spider gear case, spider gears. The other side is the reverse of this list. What we noticed is the worn out cross hatch pattern that used to be on the spider gear case. I R&Red it by bringing it back with some 200g sandpaper. I had also looked at the condition of the clutches and the drivers side clutch had less "gription" material than the passenger side. We decided that flipping them to opposite sides and facing the most material inward would help the life that was left in these clutches. Makes sense that the drivers side wears out faster since it carries more weight. Perfect world, we would have ordered new clutches and the 25% slip would have been back to full. Pressing the new bearings required a bit of finesse on the part of my partner. What he did was to freeze the two parts that the bearings slid on to for 20 minutes and I prepped the new bearings by heating them prior to taking the case parts out of the freezer. Parts came out, we slapped the bearings on with minimal effort. Then it was just a matter of filling the LS case with the clutches washers gears and torquing the allen head screws to the appropriate torque.

The differential case, well that was a whole different story...

It was cleaned up (degreased) using a bit of gasoline and a hard brush to get all the accumulated junk off the outside of the case and the Alpina diff cover. It would have reduced the cooling ability of the case and the cover since the gunk and junk actually acts like an insulator while it is on the case parts. Moving the input shaft around we noticed tightness of the pinion turning on its bearings. We thought it was grit and junk from the gasoline bath and sprayed it with parts cleaner. It no longer felt "crunchy" but now it squealed when the input shaft was turned. Pushing down on the case only made it louder.

My friend bit the bullet and we removed the pinion from the case.

What did we find? Well, what was in there was one corroded and worn bearing. Not the one that rides close to the gear part of the pinion but close to the snout of the diff case itself. The cage that held the rollers on was all loose and the bearing case was rusted on the pinion shaft. There was no evidence of water in the gear oil and the input shaft seal was sealing the case. So how did this bearing rust you may ask? Very good question. Between two points of view and eliminating the obvious we came up with the following:

Differentials heat up while in use and LS diffs heat up even more. When a hot object cools it tends to pull more moisture from the air and this moisture condenseses quicker on the cooling parts as opposed to items that are already outside and have heated/cooled gradually and have not collected as much moisture. If one looks at the cross section of the diff the snout tends to ride a little higher in angle and all of the gear oil is down in the main area where the ring gear and ls case sit at. With the Alpina cover the pooling of the diff oil twoards the rear is all the more pronounced even with the increased capacity. Kind of makes a person wonder if we are filling the diffs with enough oil. The two forward bearings are supplied with oil but only when the ring gear picks it up and throws it forward much like a paddle wheel. The term "windage" or "roping" is used here. This is what keeps the forward bearings lubed during normal operation. But when all of this oil pools during period of non-operation the forwardmost bearing is what is left high and dry and moisture condenses on this before anything else. Rusting is faster here as well since the part has a machined surface and has not been protected with any type of coating(graphite, teflon, ect.). During operation any moisture that dripped into the gear oil is evaporated by the heat and expelled through the vent hole but the rust is still on the pinion and bearing and accumulates over time. Every time the bearing turns the bearing will break off the rust and grind itself down until the next rust session. This goes on, Ad Nauseum. Until the part fails or until someone is curious enough to see what is happening and hopefully catch it in time.

Environmental conditions for this area (Colorado) is at a very arid 6100 feet above sea level with minimal moisture in the air during the year. Other than graphite coating the non moving parts/pinion surface ( not too many in there) I am not sure what else could be done to inhibit the rust on the parts.

So after looking for the appropriate bearing the surface rust on the pinion itself will be taken down with some crocus cloth, measured for tolerance, and the new bearing fitted. Another crush washer will be ordered so that the pinion will have the correct lash and the input flange torqued to spec. Everything else is the simple reversal of disassembly.

Written by RDAvena.