Talk to Me About LSDs (128i)

[blnk-128]

I'm finally going down the road of installing an LSD this winter, and am currently debating between the tried and true DiffsOnline approach vs installing a Wavetrac myself.

The reasons for doing it myself are I already have a spare pumpkin, and stand to save between $1500 - $2000 by doing the labor. From what I've been able to gather and thanks to some other DIYs posted, I feel relatively comfortable with swapping the ring gear, ordering C-clips to set the backlash, and installing new bearings onto the outputs of the diff carrier.

My main concern going into this is "I don't know what I don't know".

Can anyone confirm backlash specs for an e82 128i / e90 328i automatic differential? I've seem some numbers floating around (.0024 - .005) but no source material. My backup plan is to measure the preexisting backlash and go from there.

Regarding preload, given there are no clutches in the Wavetrac unit, is preload something I need to be worried about?

I'm leaning towards the DIY route as if I screw something up I'll likely notice excessive clunking or poor performance and can either re-do it or send it off to someone to do the job right.

I'm open to any and all advice.

Posts I've found informative:
1) Helical Diff Install: https://www.1addicts.com/forums/show....php?t=1835048
2) 3.23 -> 3.73 Diff Swap: https://www.1addicts.com/forums/show....php?t=1418770
3) Diff Disassembly:

4) Bearing Race Install Video:

5) Finned Diff Cover: https://www.1addicts.com/forums/show....php?t=1254502

Parts:
Wavetrac Diff: https://autotech.com/i-30498917-wave...88k-r3-15.html
Shims: https://autotech.com/i-30499234-188k...62-4-00mm.html
Replacement Bearings: https://diffsonline.com/products/car...43960920932650

[4JawChuck]

Replacing the ring and pinion in any differential is pretty straight forward and not that hard to do, can be done in the car as well.

One trick I have learned doing dozens of hypoid drive systems is the machined sides of the ring gear and pinion form a parallel surface that is machined when the gear is hobbed. This flat plane can be used to eye ball the install of both gears to get it very close to perfect alignment and depth which usually only requires a side clearance adjustment to refine the back lash.

In other words a straight edge placed on this machined surface should be flush on both gears…no light showing when checked with a straight edge.

Simple concept that most mechanics never are shown or realize unless you have actually machined the gears themselves which is rare today.

People tend to over complicate the setup of hypoid gears because they don’t understand the relationship of how they are made which makes life so much easier when doing the first setup and checking backlash…pretty simple, these two surfaces must be parallel and flush when checked with a straight edge.

Don’t think I have ever seen anyone mention this in any manual or description on how to do it.

[blnk-128]

I've never heard of the straight edge approach, seems like a smart sanity check. Luckily the pinion doesn't have to come out, and because the ring gear bolts right on the new carrier it "should" be at the correct depth. As you mentioned, the only requirement will likely be setting backlash with side to side adjustment via the c clip shims.

[4JawChuck]

Of course the hardest part with automotive style differentials is accessing the pinion to ring gear interface which can be complicated to visualize due to tight clearances, might need a little ingenuity with a small straightedge and mirror or scope to get in there.

Have you considered changing oil weight to increase drag on the side gear assembly which tends to make the differential act more as a posi?

For example my 128i was a PITA in the winter doing the one wheel peel when taking off from a start unless you gave it a lot of wheel spin to get the e-diff/brakes working to make both tires spin especially in a few inches of snow which my 5.7L Challenger has no issue with.

Switching to 75/140W gear oil sure improved the action even in the summer and world's of difference in the winter, on low mileage axles the effect is substantial enough that it may make you reconsider putting an actual posi in.

If your racing you need a posi of some sort of course, give the heavy oil a try first if this is just for the street...you might be pleasantly surprised.

In the old days we called this a poor mans posi, a refill with straight weight 140W. I have not been able to find this old gear oil spec because everything is synthetic these days but the multi grade 75/140W I've had good luck with in my car.

[spidertri]

I bought just the pumpkin and installed it myself two years ago. It was a straightforward process and not a bad DIY. I reused my existing bearings at the time thinking that I might go to a different gear ratio in the future but that hasn't happened yet.

For the wavetrac you're considering, it does have clutches inside, they are just smaller than normal and provide some lockup in certain situations. Do some reading on those, I've seen comments and threads from some of the faster track guys saying the clutches don't last very long.

I worked with Florian from limitedslip.de and he built my LSD to my specs, including the preload setting. Then he just shipped it over and I installed it. I can look and see if the backlash numbers were on the datasheet he sent.

You should definitely measure the backlash before you take it apart, always good to know where you're starting at.

[cerealwars]

Originally Posted by 4JawChuck

One trick I have learned doing dozens of hypoid drive systems is the machined sides of the ring gear and pinion form a parallel surface that is machined when the gear is hobbed. This flat plane can be used to eye ball the install of both gears to get it very close to perfect alignment and depth which usually only requires a side clearance adjustment to refine the back lash.

In other words a straight edge placed on this machined surface should be flush on both gears…no light showing when checked with a straight edge.

Hey Chuck, how does this work? I'm having a hard time visualizing where you are talking about. Which parallel surfaces? There are several machined faces on both parts - the most obvious of which are the teeth. Are you suggesting to check the airgap between the tooth mesh? Or like, the back face of the pinion and the OD of the ring gear?

I have rebuilt several diffs and would love to know this shortcut for checking. One shortcut I can recommend is to grab the pinion and give it a rotational shake. Feel the stock backlash before you take it apart. Spidertri's advice to measure it is even better.

I have made assembly errors with these in the past, and after feeling the stock interface it is immediately obvious whether you got it right or not.

[4JawChuck]

Here is an image showing the outer edge of the ring gear as well as a pinion in place to show the relationship. What’s not shown is the outer beveled machined edge surfaces are parallel and flush when correctly setup.

I use this method to first set up the pinion depth and then set backlash with the ring gear side to side. It can be difficult to see this in an assembled axle housing unless you’re building something like a 9” Ford removable centre section where you have all kinds of visual access to see.

The most common problems I have seen with hypoid differentials done by other mechanics that have failed is they almost always are getting the depth wrong and interpreting the pattern incorrectly.

Caveat I have never done a BMW differential so not sure what kind of access there is but this is the rule of thumb, if this beveled edge is offset from each other the depth is wrong.

It’s hard to describe but easy to see when assembled correctly.

[AmuroRay]

Winners don't use drugs.

[blnk-128]

Appreciate all the input.

4JawChuck I hear you on adding the thicker fluid, as it stands I'll probably be running something like that with the LSD.

spidertri Good to hear someone else went the DIY route for install, if you have those backlash numbers it would give me some peace of mind. Speaking to Diffsonline it sounds as though they've never had one fail on track, though so much of this stuff is anecdotal. For performance he 100% recommends clutch types, but conceded they need rebuilds every 4-5 years with heavy track use.

AmuroRay No money left over for drugs after this LSD.

[blnk-128]

Wavetrac Diff Carrier:

• PN 30.309.175WK
• Cost $1064
• Link Mammoth Racing Wavetrac Diff

3.73 Diff Core:

• Cost $120

ARP Ring Gear Bolt Kit:

• PN N-M12-150-24K
• Cost $78
• Link Turner ARP Ring Gear Bolts

BMW Axle Lock Ring (2x):

• PN 33131214961
• Cost $6
• Link Turner Output Axle Lock Ring

BMW Differential Output Flange Seal (2x):

• PN 33107505604
• Cost $18
• Link FCP Output Flange Seal

Autotech Snap Ring Shims(3x):

• PN N-472-090K
• Cost $62
• Link Autotech Snap Ring Shim

Loctite 270:

• PN N-472-090K
• Cost $15
• Link Red Loctite

Permatex 82180 Ultra Black:

• PN N-472-090K
• Cost $8
• Link Black RTV

Magnetic Drain Plug:

• Cost $47
• Link Dimple MDP

Total Cost (so far): $1418

[mkeiser]

excited to see your progress, go get 'em!

[spidertri]

Hey, sorry, just remembered this and found my install notes. I was told the backlash should be between .003" and .005", which matches up pretty well with the numbers you had already found.

[MAMSport]

Not sure if you have already ordered the Wave Trac? I have on in my car (previous owner installed). It has a 3.46 ratio, up from the factory 3.32 ration with the manual transmission, as you know. The highway revs are quite high, and while fuel economy isn't my priority, it is a bit loud with the performance exhaust. Also, when it is slippery out (snow/ice), I find that I am better starting from a standstill in second gear. I do like the extra 'dig' most of the time but am not sure I would want 3.73. I'd like to have the 7-speed DCT to bring highway revs down a bit!

Just something to consider.

[blnk-128]

10" Snap Ring Pliers

• Cost $55
• Link Channel Lock Snap Ring Plier 10"

1/4" Beam Torque Wrench

• Cost $25
• Link 1/4 Inch Inch Pounds Torque Wrench

19mm 12pt Socket

• Cost $7
• Link 1/4" Inch Pounds Torque Wrench

Backlash Gauge

• Cost $40
• Link Backlash Gauge

3 Jaw Bearing Puller

• Cost $20
• Link Bearing Puller

Other: Hammer, Chisels, Gear Tooth Paint, Vise, Dremmel (for getting behind access behind bearing), Impact, Angle Grinder (for shaving down socket)

[Donkeyshins]

Just as an FYI, the 33117516280 finned diff cover is NLA from the factory. I see one that is used on eBay from Portugal, but you'll be paying close to $300 for it.

What I did (because I'm a wuss), is I bought an E60 diff complete for $260 and will use the pumpkin as my core with DiffsOnline. Tip: if you find one and the breaker includes the VIN, use that w/ RealOEM to make sure you are getting a 188K diff rather than a 215 diff.