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Help IDing a diff


Georges
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Technically, it’s a 3.91 (43/11), which is an e21 ratio. A 3.90 — I’ve not yet seen a limited-slip version of the fairly rare 3.90 — has 39 and 10 teeth on the ring and pinion, respectively. The short-neck 3.90 is probably only found on 49-state versions of the U.S.-spec 1976 2002.

 

The “10” below the “11  S  43” represents October. There’s probably a single digit stamped below the “10”, representing the final digit of the year of manufacture, e.g., a “9” for 1979, a “0” for 1980. Below is the e21 LSD currently in my ‘76, a 3.64 LSD unit (“11  S  40”) manufactured in May 1978 (“5” over “8”).

 

I cannot see the side covers in your photos, but I’m guessing they are 6-bolt side covers, simply because they are most common on 3.91 LSD’s. The covers on pre-1979 side covers were secured with 4 bolts. Although 1975-78 e21 LSD’s exist, they are relatively rare, particularly in the U.S., and many have 3.64 ratios, 40 and 11 teeth.

 

Regards,

 

Steve

 

 

 

F9D9CDFB-EC57-42CC-90C2-7023A8C4F04C.jpeg

2B30E7A7-C462-4C3C-915C-7E97A31553C5.jpeg

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14 hours ago, Conserv said:

Technically, it’s a 3.91 (43/11), which is an e21 ratio. A 3.90 — I’ve not yet seen a limited-slip version of the fairly rare 3.90 — has 39 and 10 teeth on the ring and pinion, respectively. The short-neck 3.90 is probably only found on 49-state versions of the U.S.-spec 1976 2002.

 

The “10” below the “11  S  43” represents October. There’s probably a single digit stamped below the “10”, representing the final digit of the year of manufacture, e.g., a “9” for 1979, a “0” for 1980. Below is the e21 LSD currently in my ‘76, a 3.64 LSD unit (“11  S  40”) manufactured in May 1978 (“5” over “8”).

 

I cannot see the side covers in your photos, but I’m guessing they are 6-bolt side covers, simply because they are most common on 3.91 LSD’s. The covers on pre-1979 side covers were secured with 4 bolts. Although 1975-78 e21 LSD’s exist, they are relatively rare, particularly in the U.S., and many have 3.64 ratios, 40 and 11 teeth.

 

Regards,

 

Steve

 

 

 

F9D9CDFB-EC57-42CC-90C2-7023A8C4F04C.jpeg

2B30E7A7-C462-4C3C-915C-7E97A31553C5.jpeg

What about this one?

E8BDF08D-AB2F-4290-939C-6C968400857F.png

4244F5A0-F68E-4BB6-ADF8-12D717972F16.jpeg

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18 hours ago, jibarnes said:

What about this one?

E8BDF08D-AB2F-4290-939C-6C968400857F.png

4244F5A0-F68E-4BB6-ADF8-12D717972F16.jpeg


I dunno! If we take “11  S  40” to be pinion and ring gear teeth numbers, with “S” for limited slip in between — the usual format — we’re left with… maybe… “3  7” out of the ordinary place for month and year. Sometimes the month or year digits are “laying down” — turned 90 degrees from the tooth numbers — but I’ve not seen this stamping previously: month and year adjacent to each other and above the tooth numbers. This is factory, but not the norm!

 

Are the side covers 4-bolt (pre-1979) or 6-bolt (post-1978)? If the diff wasn’t installed, I’d next look to see if there is a casting date on the top rear of the housing. Various dating conventions appear on these casting dates, and many housings, admittedly, do not have the casting dates on the top rear of the housing.

 

Below, in the first photo, is the housing for the e21 LSD currently in my ‘76. It was cast April 28, 1978 (“28478”). The differential, as I showed previously, was assembled May 1978 (“5” over “8” in the second photo). Note that this is an “early e21” LSD with 4-bolt side covers.

 

Regards,

 

Steve

 

 

8E5A8F02-65A3-4A29-A5D5-D265CF96AFEF.jpeg

881B47ED-2B2D-4B47-B1B2-1D6B208547AB.jpeg

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1 hour ago, Conserv said:


I dunno! If we take “11  S  40” to be pinion and ring gear teeth numbers, with “S” for limited slip in between — the usual format — we’re left with… maybe… “3  7” out of the ordinary place for month and year. Sometimes the month or year digits are “laying down” — turned 90 degrees from the tooth numbers — but I’ve not seen this stamping previously: month and year adjacent to each other and above the tooth numbers.

 

Are the side covers 4-bolt (pre-1979) or 6-bolt (post-1978)? If the diff wasn’t installed, I’d next look to see if there is a casting date on the top rear of the housing. Various dating conventions appear on these casting dates, and many housings, admittedly, do not have the casting dates on the top rear of the housing.

 

Below, in the first photo, is the housing for the e21 LSD currently in my ‘76. It was cast April 28, 1978 (“28478”). The differential, as I showed previously, was assembled May 1978 (“5” over “8” in the second photo). Note that this is an “early e21” LSD with 4-bolt side covers.

 

Regards,

 

Steve

 

 

8E5A8F02-65A3-4A29-A5D5-D265CF96AFEF.jpeg

881B47ED-2B2D-4B47-B1B2-1D6B208547AB.jpeg

It has 4 bolt side covers, I bought it years ago on eBay together with a 177 steering box in the same auction. Seller said they were removed from a turbo. Dug through old pictures after I had it rebuilt and found date (74) like you describe. So 11/37=3.36 S, 40 has to do with date or %?

 

Thanks,

 

James D5E3B614-DFF1-49E1-A664-10F96240C16D.png


 

 

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22 hours ago, jibarnes said:

It has 4 bolt side covers, I bought it years ago on eBay together with a 177 steering box in the same auction. Seller said they were removed from a turbo. Dug through old pictures after I had it rebuilt and found date (74) like you describe. So 11/37=3.36 S, 40 has to do with date or %?

 

Thanks,

 

James D5E3B614-DFF1-49E1-A664-10F96240C16D.png


 

 


Wow. That is a rare bird, indeed! Housing cast January 19, 1974 (“191” and “74” cast into the top rear of the housing), also pointing toward a turbo “origin story”. 🙌

 

And sold with a quick ratio steering box to boot! 👍
 

Regards,

 

Steve

 

 

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After fifty years of being in the hands of various BMW owners, all wanting to "upgrade" their car, play it safe and DON'T TRUST  what's stamped/painted on the outside of the diff case.  Disassemble it,  

I've got four diff's for my car.  They're all stock.  Trust me;).

 

Mac

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5 hours ago, mac said:

After fifty years of being in the hands of various BMW owners, all wanting to "upgrade" their car, play it safe and DON'T TRUST  what's stamped/painted on the outside of the diff case.  Disassemble it,  

I've got four diff's for my car.  They're all stock.  Trust me;).

 

Mac

No need to dissassemble, just pop the rear cover off and count the teeth on the ring gear.

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