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Differential swap Q's


rjd2

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Hey folks-so I have the rear subframe removed from my car, and am doing a complete refresh back there. I have what I believe is a rebuilt LSD to put in. To confirm, the LSD will turn both sides in the same direction if I spin it manually, correct? The rear mounting plate of the diff on the LSD is not the same as what's coming out of the car. My plan was to swap them, since the mounting tabs don't exist on the new one. Anything else I should know while I'm doing this job? I have new bushings en route for the plate that mounts to the underside of the car, as well as the studs at the front of the subframe. The old diff is on the left, new is on the right. 

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Verify the diameter of the bolts you currently car has on the diff output flanges there are 8mm on the early cars and 10 mm on the cars from 74. You may want a spacer/diff flange bolt kit for the differential output flanges as the diff is slightly narrower than the 2002. Will save the life of the CV joints, or use a 320I inner CV joint. Don`t forget to use limited slip oil when replacing the oil in it. Verify break away torque there are various forum posts relating to that in this forum or online.

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Maybe it's the picture angle, but the first 2 pictures look like a early longneck differential.  Is this the differential you are trying to install?  The rear cover on the first two pictures looks like the cover in the right differential in pictures #4.  The right differential in pictures #3 and #4 is the LSD (Pumpkin, vs Open gearing).  If you are trying to install the longneck differential, you need a longneck differential subframe, (and a different driveshaft).  If the clutch pack on the LSD is badly worn, spinning the input flange won't necessarily spin both output flanges in the same direction.

 

I maybe wrong, need more pictures.

 

Mark92131

 

 

open-vs-limited.jpg

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1970 BMW 1600 (Nevada)

 

 

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thanks folks. FYI: pic 1=old diff, pic 2=new diff.

 

I am also doing a 5 speed swap(my goal was to build a knock down, drag out perfect "stock-ish" 2002). I have a spare drive shaft, but also the shortened one for the 5 speed conversion. bushings ordered. i have a hydraulic press, and have roasted things previously so hopefully i can emerge victorious with fire and pressure on my side.

 

sorry to harp on this. is swapping the rear diff plate the way to go? the "finned" rear plate on the LSD doesnt have the mounting tabs.

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The more I look at those posted pictures, the more confused I get.

 

As you scroll down

 

Picture #1 (bottom of an early "long-neck" differential, drain plug)

Picture #2 (top of an early "long-neck" differential)

Picture #3 (2 "short-neck" differentials, one on left open, one on right LSD)

Picture #4 (2 "short-neck" differentials, one on left open, one on right LSD), both have two fins on the top, matching cases and you can see the input flange is a short one.

 

If the 4 mounting holes on the LSD differential line up with the 4 holes on the sub-frame, swap the rear mounting plate from the left "open" differential and you are good to go.  You just then need to deal with the output flange holes and spacers as @Mikesmalaga72pointed out.  I still don't understand the pictures of the 'long-neck" differential?

 

Mark92131

 

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1970 BMW 1600 (Nevada)

 

 

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Wait.  

 

Before you go swapping everything out,  check out your 5-speed.  To rebuild them is $5-$10K USD, and only one person will do it; he is not giving up any secrets, is very picky about what a rebuildable core is and will take many months to do it. 

 

The problem is the layshaft bearings, which are unobtainable.   From what I can tell (and this is a wild guess), the transmission is taken apart, the case halves are put back together, the new bearing bore is cut (line bored), a new bearing is put in, and the transmission is put together. 

 

Sounds simple, but it is mind-blowing how hard it is. Assuming you can find the machine and tooling, you would need someone willing to try to do it, that is, after you have researched and have a bearing in hand.   This process may or may not include having to spray weld the layshaft itself and re-size the inner race. After re-sizing it may need to be reheat-treated and nitrated,  which is a very special treat in itself.

 

I just tucked my RX8 6 speed away for a bit of a winter nap, but I will need to get it out more sooner than later because my existing 5-speed is whining, and the backup I have in the attic is likely in the same shape, for the same reasons. 

 

The person who makes the adaptor plate for the RX8 took his out, and as far as I can tell, nobody is running it. It smells like it is possible to marry up the transmission, but I suspect that there is an unfixable problem with driveshaft fitment/geometry.    

 

 

 

 

 

"Goosed" 1975 BMW 2002

 

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8 hours ago, Mark92131 said:

The more I look at those posted pictures, the more confused I get.

 

As you scroll down

 

Picture #1 (bottom of an early "long-neck" differential, drain plug)

Picture #2 (top of an early "long-neck" differential)

Picture #3 (2 "short-neck" differentials, one on left open, one on right LSD)

Picture #4 (2 "short-neck" differentials, one on left open, one on right LSD), both have two fins on the top, matching cases and you can see the input flange is a short one.

 

If the 4 mounting holes on the LSD differential line up with the 4 holes on the sub-frame, swap the rear mounting plate from the left "open" differential and you are good to go.  You just then need to deal with the output flange holes and spacers as @Mikesmalaga72pointed out.  I still don't understand the pictures of the 'long-neck" differential?

 

Mark92131

 

The first 2 pictures are just the bottoms of the 2 different short neck differentials. Long neck differentials have the bolts spaced much farther apart.

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so the dimensions of the two diffs appear to be the same. the only differences are (as stated) the input flange on the LSD looks like 10mm bolt size as opposed to 8mm. the guts of the LSD look very clean.

 

thanks on the 5 speed. i had it gone thru by my local tranny shop-they said the guts all look good. im gonna throw it in and see what happens, as i've already bought the conversion parts. thanks!

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20 minutes ago, rjd2 said:

so the dimensions of the two diffs appear to be the same. t

Stated in a post above, the width of the LSD is narrower, so the half shafts need either a shim plate or a combination of half shaft parts from an E21, search for a post on this.  Some run as is but the end play is lost and when the suspension is extended, the cv joints suffer. 

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A radiator shop is a good place to take a leak.

 

I have no idea what I'm doing but I know I'm really good at it.

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11 hours ago, 2002iii said:

The first 2 pictures are just the bottoms of the 2 different short neck differentials. Long neck differentials have the bolts spaced much farther apart.

 

The Differential in the first 2 pictures is not pictured in the second 2 pictures.

 

Mark92131

Long Neck.jpg

Short Neck.jpg

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1970 BMW 1600 (Nevada)

 

 

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Once upon a time someone out in CA made some very nice output flanges that made up the width difference between the narrower E21 diff and the 02 diff, allowing one to bolt in an E21 LSD (with the 02 rear cover) and still use the original 2002 axles.   They appear to be turned from a single piece of metal vs welded up--a very neat job, and they use 8mm bolts.

 

Dunno if they're still available; mine were given to me 20+ years ago so don't know the manufacturer.  

 

mike

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'69 Nevada sunroof-Wolfgang-bought new
'73 Sahara sunroof-Ludwig-since '78
'91 Brillantrot 318is sunroof-Georg Friederich 
Fiat Topolini (Benito & Luigi), Renault 4CVs (Anatole, Lucky Pierre, Brigette) & Kermit, the Bugeye Sprite

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3 hours ago, Mike Self said:

Once upon a time someone out in CA made some very nice output flanges that made up the width difference between the narrower E21 diff

Check with with Aardvark.

A radiator shop is a good place to take a leak.

 

I have no idea what I'm doing but I know I'm really good at it.

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