Swapping rear differential gear ratio sizes...

[jfalcon]

So, I'm dealing with this rear differential thing. I have a 4.8i that RealOEM says should be a 3.91 gear ratio according to the VIN. I couldn't find any images of one that have show the same markings as the one I pulled off my vehicle.

In fact, the numbers off the one I pulled show for a 3.46 gear ratio according to the numbers on the casing - I'm not sure if that is normal since I cannot find images of a 3.91:1 differential - and the one I was wrongly sold was a 3.15 gear ratio for a F15.

So obviously the F15 one isn't going to work (it's for sale if anyone is interested.. )

There is a 3.0 being parted out near me that has a rear differential out and ready to go. Question is: What would be the risks and gotchas here? Ultimately, I just need to get the vehicle back on the road for a while as I plan to switch to a hybrid when I can, but need a little advice on what way to move forward with this and spending even more money.

Thanks!

[Henn28]

Just saw your other post so I’ve edited the below…I’ve got a 4.4 but the diffs look the same (except for the rear cover), so I’m assuming the axles are held in with snap rings. On the e53 though their are stub axles that pop into the diff first, then the drive axles bolt to these. Same principle tho.

My first thought is that they will fit, but the snap rings are being difficult. Which they can be. If your axles use the same snap rings that the 4.4 diffs those can be a challenge to “pop” into the carrier, especially when the diff is in the car. If your axel splines are smoothly meshing with the diff splines, but won’t bottom-out with a firm “click”, the. I suspect you just haven’t gotten the snap rings to click into the grooves in the diff.

The case looks the same as the “188k” medium case as the 4.4 or 4.6, but the e53 has stub axles that pop in and out of the diff and which the drive axles bolt to. These are easier to pop into the diff with a rubber mallet. Assuming your axles seat with snap rings, pull one out slightly, turn it 30 degrees and try to reseat it…rinse and repeat. But beyond your issue perhaps stemming from just not getting the snap rings seated, have you pulled the rear cover from that diff and ensured there isn’t any damage inside? It sure looks like the splines are meshing but the snap rings just aren’t popping in, however, I suppose the diff splines could be full of crap from a failure.

As far as other options, only thing that really matters is that the front and rear diffs are the same ratio, or final drive. The front and rear wheels can’t turn at different speeds. Not sure why the part number isn’t on your diff, although it may have been on a sticker when assembled new, but if you just can’t seat the snap rings, you should pull the rear cover on your diff that was working and count both the ring gear and the pinion gear teeth. Dividing the ring gear number by the pinion gear number willl tell you exactly what the final drive ratio is that was working on the car and go find a replacement diff. Since it was working, you can assume that the front diff is the same ratio.

Assuming your diffs had not been swapped previously (did you buy the car used?), I’ve got to think there are plenty of 4.8 rear diffs on eBay you can pick up for fairly cheap. Then you could do the seals on it while it’s easy (not installed) and the. swap it in. If it has been swapped out by a previous owner, maybe to have a different final drive ratio, it could be from a 4.4 or 4.6. The cases look the same, but the rear covers would have had to have been swapped as your pics show a suspension bolt on the rear cover(?). The 4.4/6 has all three suspension points on the case. I do think it highly unlikely that someone went to all the ass-pain of swapping both diffs on a 4.8, just to get a different final drive ratio.

Beyond the above, you could buy eBay front and rear diffs from the same car and swap them in, but the front diff is a big job.

[andrewwynn]

3.91 correct for 8 cyl. Some people that never drive on the freeway have put the 6 cyl dif on the 8 for a big boost in ass dynamometer.

You need to get the correct rear diff.

I'm not sure when they changed but prob with the n55/n63 motors.

3.91 and 4.1 are the e 53 sizes.

You likely share the same diff from e 53 which would give you a lot more options. What is last 7 of your vin?

[andrewwynn]

Sadly they must have changed something.

https://www.realoem.com/bmw/enUS/par...&q=33107602986

Looks like just one part number and no shared older models.

[Henn28]

Definitely different than the e53 then. How are those output flanges held on? No bolts or clips.

At any rate, axle instruction notes below…definitely a c-clip involved here, so my money is on that, and see the note about deformed splines. Take the cc clip and see if they seat all the way into the diff.

Installation note:
High installation forces indicate that the output shaft spline teeth are damaged or deformed!
Check gearing, replaced damaged parts if necessary.
Check dust plate (1) for damage, renew if necessary.
Replace circlip (2).
Coat contact surface (3) of output shaft with approved final drive oil.

[andrewwynn]

I always test fit CV axle without the c clip to make sure it'll bottom. Position matters need to have the opening either 12:00 or 6:00 i forget which.

Unrelated to the original topic here of which diff fits and pre LCI e 70 only has one part number.

[jfalcon]

I did look up the barcode on the old rear diff that was on the rear cover and that seems to match up for the 3.91 rear differential - I am assuming that the internals are the different for the 3.91 despite the pumpkin itself having a identifier that is for the 3.46 or 4.44.

The rear diff I was sold was for a F15 series which while it does bolt up to the car, the splines don't lock in - in fact you can see a gap on one side where the output shaft has maybe a cm of daylight between the dust shield and the pumpkin itself signifying that the pumpkin itself is too small. And there is so much play that there is no way that would work to deliver power to the wheels without it just tearing itself apart again.

It is actually getting a little difficult to find these 3.91 rear diffs that aren't for other vehicles as the one major difference I see in them are these spacers or whatever on each output shaft side that then bolt into what I assume are the output shafts themselves which means it would need different output shafts as they bolt completely different than just the splined version of them. Of course there are a plethora of M rear differentials, which wouldn't work again with this setup without changing out the output shafts along with whatever else.

[andrewwynn]

https://www.ebay.com/itm/23348602805...mis&media=COPY

Roughly $1400 for a refurbished with warranty if your old one case in good shape for core return. ($400).

[jfalcon]

Originally Posted by andrewwynn

https://www.ebay.com/itm/23348602805...mis&media=COPY

Roughly $1400 for a refurbished with warranty if your old one case in good shape for core return. ($400).

Just pulled the trigger on one for about $800 with only 85k on the clock.

[jfalcon]

What also has me a little tearful is that there are two 4.8i E70's of the right years being parted out just across the Canadian border. Getting them imported now that tariffs are now spiked up has me a little sad.