Differential Replacement

Over the past weekend, I replaced the differential in my 1976 2002. I spread the project over two days, and it was relatively straightforward. 

 

After 189,000 miles, my original differential was leaking badly from every seal, and the only real fix for that is a complete rebuild. Rather than trying to find someone who could tackle the existing unit, I decided to move on to a 320i differential, a fairly common 2002 upgrade. I was fortunate to find a professionally rebuilt 320i limited slip differential on eBay, for about $500; the rebuild had been performed by Aardvark Racing (Bonita, California). As I understand it, units like these came standard with the 320i “Sport” package, and carried a limited slip lockup percentage of 25%, a good number for street use. For the 49-state cars of the 1976 model year, BMW changed the 2002 rear end ratio to 3.90, from the 3.64 that had been standard in previous years. The 320i diff has the same ratio, and I am content to stay with that. Yes, the engine revs a bit high at turnpike speeds, but I am not doing all that much highway travel these days, and I find that the 3.90 ratio is actually a fine setup for country-road touring—especially with the enhanced low-end torque produced by the Metric Mechanic 2200 Sport engine that I installed a few years back. And I can always do a five-speed conversion if and when my transmission eventually gives out. 

 

As many of you know, there are several minor modifications required in this differential swap. The cases are virtually identical overall. In the 320i diff, however, the outboard faces of the output flanges are about 12mm (give or take a millimeter) closer together than those of the stock 2002 diff. Spacers of appropriate thickness are therefore necessary on each side: these are usually machined of aluminum, and in my case were provided by the rebuilder.
Given the spacers, longer allen-head bolts are required as well. As to those bolts: 320i half-shafts are attached to threaded holes in the diff output flanges with six 10mm bolts, whereas 8mm bolts are used in the corresponding location in 2002s. Aftermarket flanges are offered as a solution to this problem (and are typically thicker also, to make up for the width difference), but the ones I’ve seen are priced in the $400 range. In the unit I received, the existing flanges were simply drilled and tapped for six new 8mm holes, between the original 10mm holes.
Have the flanges been weakened by the additional holes? I’m going to choose not to worry about it, as I’m not headed for the track. The final mod necessary in this swap is to replace the 320i rear cover with a finned 2002 cover, and the rebuilder had provided that also. Thin paper gaskets for diff covers do not have a great track record, so I made a gasket with the Permatex Gear Case product, and expect no problems. I let the Permatex cure for a full 24 hours before putting in new Redline fluid. 

 

Removal of the differential is uncomplicated. For access, it is best to take down at least the muffler. I dropped the exhaust center section as well, since it needed replacement. The four driveshaft bolts are easy to extract, as the bolt heads have a flat side that keeps them from spinning, and one needs a wrench on the nut only. Before removing the allen-head bolts from the half-shafts, it is important to clean out the recesses of the bolts with a pick, so that the allen tool can seat fully; stripping out one of those bolt heads would be no fun whatever. Once the half-shafts are free, they can be tied up out of the way. Visual inspection will confirm that the diff will not come out unless the rear hanger is unbolted from the subframe. Once that is accomplished, and the four large mounting bolts are removed, the unit can be tipped back and slid out. It is best to have a helper for this step. I jammed a 2×4 prop under the diff while removing the last fasteners, so that my buddy didn’t have to support the weight of the beast until we were actually ready to tip it out. Here’s how things looked with everything removed. 

 

 

Once the old diff is on the bench, the rear hanger can be transferred to the new one. The bushings in my hanger fortunately looked quite good; these bushings are not sold as separate parts, so if they’re shot, it’s necessary to obtain a new hanger. It is useful to have a helper once again when horsing the replacement diff into place. Do note that in the shop manual (and likewise the Haynes manual), there is a definite order for tightening the various fasteners upon reassembly, with the driveshaft bolts coming first. There is a dark warning about possible “drumming or vibration” if this procedure isn’t followed. And it makes complete sense when you think about it for a moment: the driveshaft flange and diff input flange need to be pulled tightly together first, before anything else is snugged. This allows the diff to take up proper alignment within the float of the mounting bolt holes. It would be wise to use all new locking nuts on the various fasteners that call for them. This photo shows the rebuilt unit in place, with half-shafts ready to reattach. Buttoning things up from this point was simple and quick. 

 

 

I was very pleased on the test drive. No noise, no vibration. And no more leaks. As noted above, I took this opportunity to replace the center section of the exhaust. For whatever reason, as soon as the old section warmed up it started pinging and snapping like an aging fin-tube radiator. The new Ansa section is blessedly silent. I coated it before installation with flat black high-temp spray paint. 

 

Here is the original diff after removal. 

 

I would be happy to give this old differential, at no charge, to any group member who would like it. I believe the gearing is sound, but the unit would require new bearings and seals. From what I’ve read, that is a job best left to the experts, and the parts are readily available. This is a 3.90 standard open diff, and it could perhaps be of use to someone. Matt Pickering, are you interested? To avoid the complications of crating and shipping, it might be best if the new owner could pick up the piece at my house in Manchester, Mass. 

 

Anyone on the hunt for a 320 differential to transplant into a 2002 should be aware that rebuilt units of this sort are sold by Metric Mechanic. But they are seriously expensive, and since MM is not interested in accepting open-diff cores, you face a hefty core charge as well. By the time that shipping and conversion flanges are thrown in, you are easily looking at well over $2000. Metric Mechanic is also able to change gear ratios, but that adds still more hundreds of dollars. I think I got lucky with my eBay find. Don’t get me wrong: I’ve had a fine experience dealing with Metric Mechanic in the past, and the 2200 Sport engine is a superb street motor. But you might do much better on a diff by shopping around. 

 

I obtained my new exhaust and all the other small parts for this project from my usual parts source: Greenfield Imported Auto Parts (Greenfield, Mass.). I have been buying 2002 and 325 parts from them for many years. They know the cars inside and out, carry most things in stock, and can get almost anything they don’t have. Because they operate a repair business also, they can provide tech advice when you need it. I deal with Steve Messer, who is excellent, but that goes for the entire staff. They can be reached at 413-774-2819. My parts always arrive the next day by standard UPS. 

 

Those of you with sharp eyes may note from the photos that I have in recent years replaced both the rear subframe and the rear trailing arms, all of which fell victim to corrosion. (I no longer drive this car in the rain or snow). The subframe is not all that difficult a replacement. The trailing arms are a bit more involved, as it makes sense to do the rear wheel bearings at the same time. The procedure for that is well detailed in the manuals: the correct combination of shims must be installed in conjunction with the large tubular spacer, such that the bearings themselves are not improperly stressed when the castellated axle nut is torqued down. Some math is involved, and a digital caliper with depth measurement and metric readout is a must. As long as I was doing the trailing arms, I went ahead and replaced the tired brake backing plates also: new adjusters work far better than 35-year-old ones. I’d be happy to talk to anyone who is thinking about taking on these jobs. 

 

I am blessed to have a grease pit (installed around 1920) in one of the bays of my shop in our carriage house.
This was a godsend for the differential replacement, as the transplant would have been a bear to carry out with jackstands and creeper. If you are thinking of replacing your diff, try your best to get access to a grease pit or lift.
Good luck to all who attempt this repair.
Jay

 


 


 


 


 


 




Those of you with sharp eyes may note from the photos that I have in recent years replaced both the rear subframe and the rear trailing arms, all of which fell victim to corrosion. (I no longer drive this car in the rain). The subframe is not all that difficult a replacement. The trailing arms are a bit more involved, as it makes sense to do the rear wheel bearings at the same time. The procedure for that is well detailed in the manuals: the correct combination of shims must be installed in conjunction with the large tubular spacer, such that the bearings themselves are not improperly stressed when the castellated axle nut is torqued down. Some math is involved, and a digital caliper with depth measurement and metric readout is a must. As long as I was doing the trailing arms, I went ahead and replaced the tired brake backing plates also: new adjusters work far better than 35-year-old ones. I’d be happy to talk to anyone who is thinking about taking on these jobs. 

I am blessed to have a grease pit (installed around 1920) in one of the bays of my shop in our carriage house. This was a godsend for the differential replacement, as the transplant would have been a bear to carry out with jackstands and creeper. If you are thinking of replacing your diff, try your best to get access to a grease pit or lift. 






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