E38 Custom Rear Differential Questions

**01740iL1985** · 06-12-2016, 09:28 PM

Hi all! So my silver e38 has been running phenomenally and I decided to buy another. Must be gluten for punishment. I want to change the gear ratio in the rear and I have a few questions. I know it can get technical but I'm just looking for some guidelines right now.

I have a random e39 diff sitting around. It has a 4:10 gear ratio and is a non limited slip. So my question is.....if I purchased a LImites Slip for a 210mm case and took the gears out of the e39 diff, can I bolt it all back inside as the guts of my e38 diff? I realize the gears are tall I don't care. I would rather have somewhere in the 3:73 range but I already have the 4:10 so why not.

Any advice would be great!!!

O and yes I know I can purchase new with the ratio I want......not willing to spend $3500-$4500 for a rear....so just advice on the best way to get from a to b would be great!

**ColtonBH** · 06-12-2016, 11:33 PM

I don't know all the specifics on the e39 riffs so I may be wrong, but if I remember correctly, all the e39 different are very different from other BMW diffs. Even though both cases may be 210mm, the parts inside don't fit together for one reason or another. Like I said, I don't remember the specifics and its been a while so I could easily be wrong.

**01740iL1985** · 06-13-2016, 06:05 AM

Originally Posted by ColtonBH

I don't know all the specifics on the e39 riffs so I may be wrong, but if I remember correctly, all the e39 different are very different from other BMW diffs. Even though both cases may be 210mm, the parts inside don't fit together for one reason or another. Like I said, I don't remember the specifics and its been a while so I could easily be wrong.

Thanks! I was thinking of just trying to just use the ring and pinion gear. Seems like they would fit if that's all I wanted to use out of the e39 rear. I really should just open it up and see lol.

**TxDarth** · 06-13-2016, 03:02 PM

First the ring and pinion are the least expensive parts of your update....a LSD unit would top the list....then the set up is next and last the ring and pinion.

**blackknight530i** · 06-13-2016, 03:49 PM

Once again, people are confused with the e38 and e39 diff case sizes. The 96+ e38 and pretty much all e39s other than the M5 have 215k diffs (V8s that is), while the M5 has the 210 diff case, much like the old e32 and 95 e38. The 215 gears and 210 gears are not interchangeable and neither are the cases.

The first question for the OP is whether or not the e39 4.10 diff he has is from a 540i, or one of the I6 cars. If it's from an I6 car (528i, 525i, 530i), you can go ahead and scrap the idea, because they have a 188k case, which is not interchangeable at all.

So, what options do you have if the 4.10 gearset is a 210? Well, this has been said many times in many threads, but you can get a 95 e38 differential (has 210 gears, but open diff and is already a 3.15 ratio) and swap the gears to the 4.10 and if you want at that point, stick a 210 LSD unit in it from either an e32 (88-89 750il), e39 M5, or e46 M3. There might be others with this case size, but those are the most common. Put it all together and you can install basically plug and play into your e38. I think 4.10 in an e38 might be a bit too short of a ratio, but that's up to you to decide. Most e39 M5 guys stick with about 3.62/3.64 or so, but some have gone shorter. The point is, you still need to buy two more differentials to make your 4.10 gearset work in your car, which will end up being very pricey, though not as bad as a built-to-spec diff I guess.

I think another option you might want to consider is to get a 95 e38 differential and swap the internals from an e46 M3 into it, making a 3.62 LSD. Again, still requires buying 2 differentials, but you can sell your 4.10 to help fund it and the e46 M3 differentials are easier to find and not terribly expensive.

- - - Updated - - -

Oh, and you'll need specific output flanges to work with the LSD and your current axles as well. The easiest option there is to get the e39 M5 flanges and use the pre-2001 (I think) 102mm axles.

If you don't mind modifying the rear subframe a bit, you can also just buy an e39 M5 LSD differential and install it in the e38 by cutting and rewelding the rear diff mounts closer to the center of the subframe. Ask 007008 about that.

**01740iL1985** · 06-13-2016, 07:09 PM

Originally Posted by blackknight530i

Once again, people are confused with the e38 and e39 diff case sizes. The 96+ e38 and pretty much all e39s other than the M5 have 215k diffs (V8s that is), while the M5 has the 210 diff case, much like the old e32 and 95 e38. The 215 gears and 210 gears are not interchangeable and neither are the cases.

The first question for the OP is whether or not the e39 4.10 diff he has is from a 540i, or one of the I6 cars. If it's from an I6 car (528i, 525i, 530i), you can go ahead and scrap the idea, because they have a 188k case, which is not interchangeable at all.

So, what options do you have if the 4.10 gearset is a 210? Well, this has been said many times in many threads, but you can get a 95 e38 differential (has 210 gears, but open diff and is already a 3.15 ratio) and swap the gears to the 4.10 and if you want at that point, stick a 210 LSD unit in it from either an e32 (88-89 750il), e39 M5, or e46 M3. There might be others with this case size, but those are the most common. Put it all together and you can install basically plug and play into your e38. I think 4.10 in an e38 might be a bit too short of a ratio, but that's up to you to decide. Most e39 M5 guys stick with about 3.62/3.64 or so, but some have gone shorter. The point is, you still need to buy two more differentials to make your 4.10 gearset work in your car, which will end up being very pricey, though not as bad as a built-to-spec diff I guess.

I think another option you might want to consider is to get a 95 e38 differential and swap the internals from an e46 M3 into it, making a 3.62 LSD. Again, still requires buying 2 differentials, but you can sell your 4.10 to help fund it and the e46 M3 differentials are easier to find and not terribly expensive.

- - - Updated - - -

Oh, and you'll need specific output flanges to work with the LSD and your current axles as well. The easiest option there is to get the e39 M5 flanges and use the pre-2001 (I think) 102mm axles.

If you don't mind modifying the rear subframe a bit, you can also just buy an e39 M5 LSD differential and install it in the e38 by cutting and rewelding the rear diff mounts closer to the center of the subframe. Ask 007008 about that.

Question answered! Thank you very much. The rear I have is from a 98' 528. So won't fit. Thanks

**blackknight530i** · 06-13-2016, 07:10 PM

Originally Posted by 01740iL1985

Question answered! Thank you very much. The rear I have is from a 98' 528. So won't fit. Thanks

No problem!

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**01740iL1985** · 06-13-2016, 07:38 PM

Originally Posted by blackknight530i

No problem!

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I'm trying to get a little bit taller than 3:15 ratio but don't really wanna spend the money for a diff right now. However, I will just complete my project as is and maybe just spring for the correct and appropriate rear next year or something. 4:10 is way too tall but I wondered since I had the e39 one sitting around would it work lol. Again, thanks for the info.

**blackknight530i** · 06-13-2016, 07:40 PM

Originally Posted by 01740iL1985

I'm trying to get a little bit taller than 3:15 ratio but don't really wanna spend the money for a diff right now. However, I will just complete my project as is and maybe just spring for the correct and appropriate rear next year or something. 4:10 is way too tall but I wondered since I had the e39 one sitting around would it work lol. Again, thanks for the info.

One thing, taller ratios have lower numbers, 2.93 is taller than 3.15 for example. Just letting you know.

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**TxDarth** · 06-13-2016, 07:41 PM

I think the tallest rear the guys with e38's are happy with is 3:45, a few did go 3:65 I believe.

I can remember running 4:56's in my '57 Chevy many, many moons ago!

**01740iL1985** · 06-13-2016, 07:43 PM

Originally Posted by blackknight530i

One thing, taller ratios have lower numbers, 2.93 is taller than 3.15 for example. Just letting you know.

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I see! I was always under the impression taller meant higher ratios. Thank you for that! I sent you a PM btw about a cluster again lol.

**01740iL1985** · 06-13-2016, 07:44 PM

Originally Posted by TxDarth

I think the tallest rear the guys with e38's are happy with is 3:45, a few did go 3:65 I believe.

I can remember running 4:56's in my '57 Chevy many, many moons ago!

Haha yea I realize it sounds like a completely wrong gear ratio for an e38 but I like tinkering so figured I would ask.

**apexilude** · 06-14-2016, 02:17 PM

I created a diy in this sub do a search for limited slip diff in your e38. Pics included.

**Ralbovski** · 04-18-2018, 07:10 PM

Apexilude can you send me that diy ? Ralbovski@hotmail.com have a regular e38 2000 stage one Dinan and want to upgrade differential thanks

**Rhein Amacher** · 04-22-2018, 03:37 AM

Awesome! I can't wait to find these parts to add a limited slip to my 01' sport!

**Rhein Amacher** · 04-23-2018, 09:15 PM

Originally Posted by blackknight530i

Once again, people are confused with the e38 and e39 diff case sizes. The 96+ e38 and pretty much all e39s other than the M5 have 215k diffs (V8s that is), while the M5 has the 210 diff case, much like the old e32 and 95 e38. The 215 gears and 210 gears are not interchangeable and neither are the cases.

The first question for the OP is whether or not the e39 4.10 diff he has is from a 540i, or one of the I6 cars. If it's from an I6 car (528i, 525i, 530i), you can go ahead and scrap the idea, because they have a 188k case, which is not interchangeable at all.

So, what options do you have if the 4.10 gearset is a 210? Well, this has been said many times in many threads, but you can get a 95 e38 differential (has 210 gears, but open diff and is already a 3.15 ratio) and swap the gears to the 4.10 and if you want at that point, stick a 210 LSD unit in it from either an e32 (88-89 750il), e39 M5, or e46 M3. There might be others with this case size, but those are the most common. Put it all together and you can install basically plug and play into your e38. I think 4.10 in an e38 might be a bit too short of a ratio, but that's up to you to decide. Most e39 M5 guys stick with about 3.62/3.64 or so, but some have gone shorter. The point is, you still need to buy two more differentials to make your 4.10 gearset work in your car, which will end up being very pricey, though not as bad as a built-to-spec diff I guess.

I think another option you might want to consider is to get a 95 e38 differential and swap the internals from an e46 M3 into it, making a 3.62 LSD. Again, still requires buying 2 differentials, but you can sell your 4.10 to help fund it and the e46 M3 differentials are easier to find and not terribly expensive.

- - - Updated - - -

Oh, and you'll need specific output flanges to work with the LSD and your current axles as well. The easiest option there is to get the e39 M5 flanges and use the pre-2001 (I think) 102mm axles.

If you don't mind modifying the rear subframe a bit, you can also just buy an e39 M5 LSD differential and install it in the e38 by cutting and rewelding the rear diff mounts closer to the center of the subframe. Ask 007008 about that.

So I’ll I need to do is find a 95 e38 open 3:15 differential and an e46 M3 limited slip diff, preferably with 3.62 internals, and then take those internals and swap them into the e38 diff? Do I need to change my 01 740s subframe at all? Any custom welding?

**blackknight530i** · 04-23-2018, 09:28 PM

Originally Posted by ramacher

So I’ll I need to do is find a 95 e38 open 3:15 differential and an e46 M3 limited slip diff, preferably with 3.62 internals, and then take those internals and swap them into the e38 diff? Do I need to change my 01 740s subframe at all? Any custom welding?

No change to the subframe would need to be made for that swap since you're putting a 95 e38 diff in. All e38s use the same rear subframe, other than the fact that the SLS cars have brackets for a rear sway bar.

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**Rhein Amacher** · 04-24-2018, 02:13 AM

So I am guessing it will be harder to find the 95' E38 diff assembly than the E46 M3 3.62 LSD (which will be more costly$$) for the car. This mod is way easier to do than finding a E39 M5 LSD and modifying the subframe to fit our E38s. Why haven't more people gone the E46 M3 LSD route? Seems like less modifying?

**blackknight530i** · 04-24-2018, 06:34 AM

Originally Posted by ramacher

So I am guessing it will be harder to find the 95' E38 diff assembly than the E46 M3 3.62 LSD (which will be more costly$$) for the car. This mod is way easier to do than finding a E39 M5 LSD and modifying the subframe to fit our E38s. Why haven't more people gone the E46 M3 LSD route? Seems like less modifying?

It doesn't have to be e46 m3 diff, you could use the internals from the e39 m5 diff too. Hard part is finding 95 740 diffs and basically having to buy two diffs to make one. Most people just have the subframe modified and use the m5 diff as is.

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**Rhein Amacher** · 04-24-2018, 04:28 PM

Originally Posted by blackknight530i

It doesn't have to be e46 m3 diff, you could use the internals from the e39 m5 diff too. Hard part is finding 95 740 diffs and basically having to buy two diffs to make one. Most people just have the subframe modified and use the m5 diff as is.

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So am I looking for the 95’ e38 rear subframe or the 95’ e38 rear open diff casing with the 3.15 internals inside?

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**Rhein Amacher** · 04-24-2018, 04:31 PM

Nevermind, I think I need the entire 95’ e38 open diff casing and I would swap the internals from the e46 3.62 lsd inside. I would assume that once the lsd is out, I would install new seals and gaskets on the diff correct, as well as new gear oil?

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**blackknight530i** · 04-24-2018, 04:40 PM

Originally Posted by ramacher

Nevermind, I think I need the entire 95’ e38 open diff casing and I would swap the internals from the e46 3.62 lsd inside. I would assume that once the lsd is out, I would install new seals and gaskets on the diff correct, as well as new gear oil?

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Yes

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**Rhein Amacher** · 04-30-2018, 07:11 AM

Originally Posted by blackknight530i

Yes

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So I picked up a 95 3.15 diff from my local wrecking yard. Now will I only need a e46 M3 lsd with 3.62 gear ratio to complete the lsd set up for my 01 sport? Are there any other pieces I need?

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**blackknight530i** · 04-30-2018, 07:18 AM

I believe that's it, but I've never taken a diff apart, so couldn't tell you. I'm just passing along info I've read over the years.

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**RVAE34** · 04-30-2018, 07:58 AM

You will need new seals and the crush washer/spacer for the pinion gear. You may also need new shims for proper spacing. Lastly, you will want someone to measure the runout and back lash (I think that is what it's called). Have to make sure the ring and pinion gear teeth line up properly. When I put a 3.62 in the M5 diff when I built my LSx 427 touring, I got lucky and the stock shims worked perfectly.