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      04-10-2018, 10:54 PM   #1
minmat11
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Differential Fitment question! Help!

Hey guys. Found this 3.91 diff on eBay. Just wanted some sort of confirmation that it would for sure work on my 2.5i z4 auto before I purchase it. Please advise. Thank you!
https://rover.ebay.com/rover/0/0/0?m...2F273149497429
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      04-11-2018, 08:22 AM   #2
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Quote:
Originally Posted by minmat11 View Post
Hey guys. Found this 3.91 diff on eBay. Just wanted some sort of confirmation that it would for sure work on my 2.5i z4 auto before I purchase it. Please advise. Thank you!
https://rover.ebay.com/rover/0/0/0?m...2F273149497429
Good reference site here
http://www.bokchoys.com/differential/GearRatios.htm

Edit: I tried 3.73:1 and found it way too low. First gear was useless.
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      04-11-2018, 08:27 AM   #3
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Quote:
Originally Posted by dbarton62 View Post
Quote:
Originally Posted by minmat11 View Post
Hey guys. Found this 3.91 diff on eBay. Just wanted some sort of confirmation that it would for sure work on my 2.5i z4 auto before I purchase it. Please advise. Thank you!
https://rover.ebay.com/rover/0/0/0?m...m/273149497429
Good reference site here
http://www.bokchoys.com/differential/GearRatios.htm

Edit: I tried 3.73:1 and found it way too low. First gear was useless.
Thank you! I actually did read on that last night. I guess I'm just asking about Fitment.
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      04-11-2018, 08:46 AM   #4
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Well according to the reference site the ebay diff won't fit
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      04-11-2018, 08:50 AM   #5
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Dang it. I know I have a 3.64 diff but would like to upgrade it to 3.91 or at least 3.73. What car would share the right diff with my 2.5i? I'm not seeing any E46's that would have that
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      04-11-2018, 08:58 AM   #6
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The ebay diff isnt listed on bockchoys.
The diff is located in germany and came of a car that was never sold in the US.

The listed partnumber in the ebay ad is not correct, at least I cant find it in the ETK.
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      04-11-2018, 09:24 AM   #7
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I just read that E39 differentials can be shared with E85. Could somebody confirm this. And if it does, would it be better to use a diff from an auto? Or could I use a manual
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      04-11-2018, 10:28 AM   #8
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I got my 3.73 from a 2008 SMG Z4
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      04-11-2018, 12:19 PM   #9
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Quote:
Originally Posted by minmat11 View Post
I just read that E39 differentials can be shared with E85.
They cant, the housing is not compatible.
Maybe some internals can be swapped over from certain types, but that requires a special skillset.
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      04-11-2018, 01:03 PM   #10
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Okay could someone shed some light on this and tell me what vehicle I can get a 3.91 diff from? Again, I have an e85 2.5i Auto 5 speed
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      04-11-2018, 02:00 PM   #11
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From Bokchoy
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Last edited by Steeler; 04-11-2018 at 02:48 PM..
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      04-11-2018, 02:05 PM   #12
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Quote:
Originally Posted by dbarton62 View Post
From Bokchoy
I'm lookin at it. Would a diff from an e36, e90 or e83 work on an e85? Those are the only ones I see that have 3.73 ratios and above. No e46 has it. So I don't know that it'll be a compatible figment. Please advise!
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      04-11-2018, 02:52 PM   #13
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Actually it looks like only the 2.5 X3 has the 3.91 that should fit your E85

http://www.bokchoys.com/differential/GearRatios_E46.htm
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      04-11-2018, 03:05 PM   #14
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Quote:
Originally Posted by dbarton62 View Post
Actually it looks like only the 2.5 X3 has the 3.91 that should fit your E85

http://www.bokchoys.com/differential/GearRatios_E46.htm
Thanks! are there any other ones listed on there that would work?
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      04-11-2018, 03:09 PM   #15
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Be sure to get the 188k model, not the 168k model because its weak.
The ETK somehow lists the 168k model for certain 2.5i x3 models and 188k for others.
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      04-11-2018, 03:18 PM   #16
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Quote:
Originally Posted by GuidoK View Post
Be sure to get the 188k model, not the 168k model because its weak.
The ETK somehow lists the 168k model for certain 2.5i x3 models and 188k for others.
What do you mean by etk? or rather, how can i tell what's 168k or 188k?

Last edited by minmat11; 04-11-2018 at 03:28 PM..
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      04-11-2018, 04:15 PM   #17
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I've had little luck finding any e83's diffs. are there any other models on that list that I could use?
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      04-11-2018, 04:29 PM   #18
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ETK=ErsatzTeileKatalog (german for spare parts catalog, so basically realOEM or bmwfans.info but then 'official')

168K is a diff with a 168mm crown wheel and 188k is a diff with 188mm crown wheel.
The K alledgedly stands for 'Kurz' (short) but I'm not sure how the K and L diffs exactly differ.
I also dont know if there are optical differences between the 168k diffs and 188k diffs (without opening them up obviously).

This is a 188k diff (my old one):


And this is apparantly a 168k diff (advertized as from a 2.0L e85 z4 which have according to the ETK a 168k diff):


Also input and output flanges differ between engine models and cars. The output flanges are relatively easy to swap, but swapping the input flange requires special care because there is a crimp bus inside the diff that is crimped by the input flange nut. So swapping out those input flanges requires a good understanding and feel on how much drag/pretention a bearing must have or you have to have a very special torque wrench that measures in Ncm. Getting this wrong will destroy your diff
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