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      06-17-2021, 02:35 PM   #1
TimCSquared
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128i Blackline Helical Differential Install

This will be my first big thread, and it requires some backstory. I am a Mechanical Engineer currently pursuing a Masters Degree. I am also a member of my university’s FSAE team, and have been for 5+ years. As such, I have the unusual combination of experience, access to tools & resources, and absolutely no money whatsoever. This is reflected in my… unorthodox diff installation.
If anyone wants my input on something racing related, I’d love to share some of what I’ve learned. I mainly do suspension/chassis adjustments (on more than just Formula cars), but I have some experience in most racing-related fields.

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I bought a Helical diff From Blackline. I chose the 188L option. Getting a clear answer for which you need is difficult, but all signs pointed to the 188L for my ’08 128i 6MT. Perhaps this was incorrect – more on this later. The Blackline is a 12 planet helical diff with 6 preloaded Belleville washers in the center. From what I understand, this is essentially a copy of the Quaife unit.
You also need 2 axle seals, CORTECO PN# B1BASFSLDRWX27, $20 for the pair on eBay.
I will use Redline 75W90 Synthetic Diff Fluid, however there is a 500 mile break in period, and you change the fluid after to remove any burrs or particulate which may get ground away from the faces of the gears or gear bores. Normal stuff, really: The particulate/swarf can be lessened by more stringent surface finish tolerances during manufacturing, but is applicable to any diff. I threw in some junk 75W-90 Oil for the break-in. It took a little less than a quart from bone dry.
There are plenty of YouTube videos online describing the procedure, but I will document my experience below anyways.

Removing the Pumpkin from the car:
For a 128i non-Msport, this is easy. Obviously, you will need to jack up the car. Each CV is bolted to the output shafts using 6x bolts with an E12 head, remove these. The Driveshaft is attached to the pinion flange using 4x bolts with an E12 head, remove these as well. There is a centering bore between the flange and the driveshaft. Both the CV bolts and the driveshaft bolts are tight enough to probably require putting the car in gear/handbrake to allow you to loosen them without the wheels spinning. The exhaust hangers are bolted to the backside of the front diff bolts. For my single exhaust, there is just one, remove these. Now, support the diff using a jack, and remove the rear bolt and two front bolts. (Rear: 21 mm Wrench, 21mm Socket Front: 18mm Wrench) The front bolts are halfway under the heat shield, so I went slowly with a wrench to get them out. Alternatively, you could drop the heat shield and using a ratchet, or get a swivel head ratchet to speed things up. Lower/Drop the diff using the jack and slide out. 1-2 Hour Job.

Removing the center from the Housing:
First, make sure you can get the fill plug out. You want to figure that out as soon as possible. Remove the cover bolts (8x w/ 16mm head), and drain the “lifetime” diff fluid. You will immediately notice that this fluid is not, in fact, a lifetime fluid. As an oil breaks down due to excessive heat or heat cycles, it produces a very distinctive smell. You’ll see what I mean.
Take a hammer and punch and knock the Output Flanges out the sides. Now you need to get the seals out. I used a seal puller (thank you FSAE team), and it took all of 20 seconds. I think you can probably do this with some screwdrivers and some luck. Beneath the seals are the thickest snap rings you have ever seen, and they are not willing to cooperate. I had access to the correctly sized snap ring pliers, and they were still a PITA. They like to stick in the groove.
With the snap rings (hopefully) out, slide the bearings out the sides. Try to keep them from going inside the housing, you want to pull them out the sides. If they go in, you can get them out still, but it’s much harder. Then, push the center section to the right, and rotate the ring gear up and out.

Modifications:
With the center section out, you will need to grind a portion of the case away. This is on the Lefthand side (from driver perspective, imagining if the diff was installed), above the bearing on the sealing surface for the cover, as shown in my picture. Be careful about metal shavings or grit getting into the pinion gear, the pinion bearings, or the area behind the pinion gear. With the new center section, the ring gear will hit this part as you try to install it.
Unbolt your ring gear from the old center section. I used an impact driver, because I had one available. But the old center section has two big flats cut out where you can see the spider gears, which fit perfectly into a vice if you need to use a breaker bar. Leave two bolts threaded in by a few threads (opposite each other), hit them with a hammer, and knock the ring gear out.
Now, remove the bearing races. They are pressed onto either end of the center section. Thankfully, BMW left a few cut-away sections so you can get under them, but it is still no easy task. I kept all the left parts and right parts separate, so I didn’t mix them up when reassembling.
Here’s where things really fell apart. My diff center has the ring gear flange too far from where it should be, as it is on the stock unit. I’m not sure if that is because I bought the wrong one (188L vs 188K, vs etc.) or if this is one of those you-get-what-you-pay-for situations. Luckily, I have access to a machine shop, and have worked as a machinist for a few years. I measure the discrepancy and make a spacer to go under the ring gear, which is thicker than necessary. Reassemble, check it, disassemble, machine the spacer down, repeat. Each time I checked the backlash until I got it just right. Birds Auto and Quaife specified 0.0024” to 0.0055” Tangential Backlash. I machined the spacer until I got 0.0035”
How are you supposed to do it? Well most people will tell you that this differential cant be adjusted, but there are still ways to do it. I’ve heard of people using thinner snap rings outside the bearings and putting spacers between the snap rings and bearings. This will do the trick, but it will also move the bearings over. For small amounts, this is fine, but I needed to move over 0.2685”. The downside with how I did it is that if the spacer isn’t exactly the same thickness, the gear engagement will change as it rotates. To alleviate this, I was just very careful and measured it afterwards to make sure all was okay. Flat within 0.00015”.
It would also be a good idea at this point to check ring gear and pinion engagement with gear paint or Dykem Bluing Compound.
I also needed longer bolts, Grade 10.9 M12x1.5mm, 30mm Long, w/ Washers 2.5mm thick.

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Reassembly:
Clean the housing, torque the ring gear w/ Loctite, reinstall, put the bearings in, then the snap rings. The seals get pressed in, I used a hammer and a piece of aluminum tube. Reinstall the output flanges, they will require a few good blows with a dead-blow hammer. RTV the cover, and reinstall that too. I filled with fluid on the bench while I was at it.
Putting the pumpkin back into the car is the reverse of removal. 3 Diff mounts, 12 CV bolts, 4 Driveshaft bolts.

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Breaking it in:
First off, do a few sharp left hand and right hand turns, this will circulate the diff fluid up into the helical gears. Drive the car like a normal person for 10 miles. After that, you should be good to go, just flush the fluid in 500 miles.

Was it worth it?:
The whole process took me a total of two days, and that was with a well-equipped shop to play in. IT was a lot of hard work. Was it worth it? Absolutely.
As can be expected, corner exits are massively improved, especially for small radii. Oversteer recovery is also much easier. Launching yields more acceleration. Yada yada yada. The car feels great. It should have come with a LSD from factory.

Useful Resources:
Blackline Diff Disassembled: https://z4-forum.com/forum/viewtopic.php?t=102831
Bird’s Auto Install Instructions: https://www.birdsauto.com/sites/defa...68303%20v4.pdf
Diff # Identification: https://www.1addicts.com/forums/show...3&postcount=49
More Info about Stock Diff Configuration:
https://www.1addicts.com/forums/showthread.php?t=466621
https://www.1addicts.com/forums/showthread.php?t=935123
https://www.e90post.com/forums/showthread.php?t=1472442

In case anyone needs to make a spacer for themselves, mine was 0.2685” thick, 6.6” OD, 4.7” ID, with 10 0.5” holes spaced evenly on a 5.32” circle.
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      06-18-2021, 10:59 AM   #2
spidertri
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Very cool writeup, I was wondering what it would take to put an older LSD unit into our diffs.

I believe what you ran into with needing the spacer is the difference in low vs high ratio units. If you look at MFactory's catalog, for example, you'll see listings for 188 diffs under the 3.07 ratio and a different listing for over 3.15.
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      06-25-2021, 09:46 AM   #3
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I would echo spidertri comment about having the incorrect ratio carrier. There is a carrier break in these units for <3.07 and >3.15

It would seem you have the <3.07 unit and your diff is a above the break. In my experience numerically lower ratios will use a significantly larger pinion head, and therefore need more space between the pinion head and ring gear mounting surface to achieve the correct backlash. Since your ratio is numerically higher, you had to fabricate the spacer.

Do you have the link to the Blackline diff you purchased? That will help confirm the idea.
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      06-25-2021, 10:30 PM   #4
TimCSquared
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Link is below:
https://www.ebay.com/itm/401789956792
The seller also lists another 188L diff advertised for the 1 series:
https://www.ebay.com/itm/153446641798

The listings are identical as far as I can tell.

The different gear ratios are a superb explanation. I had never considered that. Or rather, I guess I had assumed that the spacing (between ring gear landing and pinion center-line) would be identical so switching between parts would be more convenient. This way, the thickness of the ring gear would be different depending on the pinion pitch diameter.

P.S. mad_hatter, I am classed in STX as well down in Texas. Let me know if you'll be at SCCA Nationals in Nebraska this September. I will be there (not driving the 1er).
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      06-26-2021, 03:58 PM   #5
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Quote:
Originally Posted by TimCSquared View Post
Link is below:
https://www.ebay.com/itm/401789956792
The seller also lists another 188L diff advertised for the 1 series:
https://www.ebay.com/itm/153446641798

The listings are identical as far as I can tell.

The different gear ratios are a superb explanation. I had never considered that. Or rather, I guess I had assumed that the spacing (between ring gear landing and pinion center-line) would be identical so switching between parts would be more convenient. This way, the thickness of the ring gear would be different depending on the pinion pitch diameter.
Your assumption is 50% correct. That is what manufacturers do, but there's a limit to the number of ratios they can build the carrier to accept. In the case of the 188L, BMW made one carrier for 3.08 - 3.91 and a second one for the <3.07. I noticed in the link they specifically call out 2.79 ratio and mostly diesel applications, which would all use numerically lower gear ratios (they don't need the torque multiplication, and also it will keep their engine speeds lower at a given road speed and tire size).


Quote:
Originally Posted by TimCSquared View Post
P.S. mad_hatter, I am classed in STX as well down in Texas. Let me know if you'll be at SCCA Nationals in Nebraska this September. I will be there (not driving the 1er).
It will depend largely on what the border situation and quarantine rules are, since I'm Canadian. There's a group from my region who regularly go down and invited me along whenever we're able to go again!
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