Diff ID

[nbristow01]

My car is a 1970 2002.  The diff that came in it differs from what I had in my 1974 2002.  My current car was an automatic so maybe this is why it differs.  The one that was ion the car has thinner flanges where the CVs mount and, in general, seems to be less robust overall.    Car also came with a second diff which I plan to use, which I think is from an E21?

 Great to know what they both are.  I have feeble search skills and found ten billion threads on diffs. After a blinding amount of looking I decided to ask directly.

[Mike Self]

The lower diff in your pictures is from an E21--notice that the mount on the cover end is completely different from the one off your 2002.  It's also narrower so you have to fiddle with either the flanges or axle shafts to make it work on a 2002, in addition to swapping out the mounting. 

 

Also there's a fair chance that the E21 diff is a 3.91 vs the original 3.64 on a 2002 (and also 77-79 320i's).  There should be ratio numbers stamped somewhere on both diffs; divide the smaller number into the larger and that'll tell you the ratio.  And if there's a big white S painted on the E21 case, you're in luck:  that denotes a limited slip.  

 

mike

[halboyles]

Look on the driver's side of the diff for markings that indicate the ratio and perhaps an "S" denoting an LSD.  The "S" might also be on the passenger side.  The one with the smooth back cover is an early '02 while the one on the bottom has a back cover from an e21.

 

Forgot to add pics of some examples of ratios/LSD lettering.  The last pic is of an e21 with a white painted "S" that is barely visible.  There might also be a tag on the e21 with the ratio and an "S".

 

Edited January 20, 2019 by halboyles

[nbristow01]

Thanks for the info!  The original one from the car, the `1600 shows 11  40  SO 3:64.

The E21 numbers are 11 43 so 3:91ish

 

I see no S on the E21 case.  When I turn one flange the other spins in opposite direction  so would seem to indicate a non LSD.

 

Car came with a spare set of CV shafts the guy said fitted the diff that came with it...the E21

 

Not sure what to do at this point.  I will be running a 4 speed, the motor is bring built with 9.5 JE pistons, 292 cam, has a 1.8 head (i got pistons to match it)

 

I would think the original early diff may not be up to the task of the added power and more spirited driving.  Relly think the E21 diff, with the 3.91, may be damn high up in the RPM range on road trip to Eureka Springs.

Also I have no interest in an LSD.  Did not have one in my last 02, which had a modded motor, and it was fine.

 

Looks like I may need to source a regular, later, 02 diff

 

Thanks tons guys and gals

 

[Simeon]

Unless there is an obvious problem with each diff, either one will be able to take the power (actually torque) of an engine as per your description. 

 

Dependent upon your actual tyre size, there is only 300 ish RPM difference between the two at 70 mph. 

 

I would choose whichever diff was in the best condition. Don’t feel you have to change for something ‘newer’ which may be the same as what you have. 

[nbristow01]

I run 205/50/15 tires on front and rear (not that front matters for this)  I think I would prefer to stay with the 3.64 1600 diff.  It seems in pretty good shape and spins smooth.  Now to figure out which CV shafts go with what.  Car came with 6.  2 in good shape, 2 in fair shape and 2 in crap shape.  

The 1600 diff has thinner CV mount flanges that are not threaded.  I assume this is just nuts and bolts.  The E21 has threaded flanges and thicker. 

The 6 CV shafts, 4 are same length and 2 are longer, so I assume the ones that are longer would have been for the POs E21 diff conversion

 

Good news is the 2 CVs in great shape seem to fit perfect with the 1600 diff. (I am mocking up on ground)

 

By the way, sorry for the questions.....this car came 90% taken apart and the parts where in bboxes mixed with E21 parts...so its a mix of all kinds of stuff in the boxes

Edited January 20, 2019 by nbristow01

[Mike Self]

1 hour ago, nbristow01 said:

the 3.64 1600 diff.

Actually all 1600s came with a 4.11 diff--at least in the US market if not everywhere, so you have a 2002 diff anyway.  Plenty strong enough for most any power you can extract from an M10 that isn't a Formula 1 motor.

 

mike

[nbristow01]

Mike, now that I think of it, you are right!

[TobyB]

The early diff (look for casting dates on the rear cover) IS in fact weaker than

a 6- bolt E21 diff- that E21 is the stoutest 168 case out there.  

 

That said, the ONLY things to worry about on the 3.64 diff are the spider gears-

they are rather feeble on the 2002 diffs, and lotsa burnouts will eventually gall and

finally chunk them.  

 

Easy to check- pop the rear cover and have a look inside the center section.

If the small bevel gears have smooth tooth faces, you're good to go, with some nice new

EP Hypoid oil.

 

However, if you see craters in the teeth of the gears, it might be time to consider your options.

 

t

would not run a 3.91 with a 4 speed if he wanted to drive long distances.

Edited January 21, 2019 by TobyB

[Simeon]

Toby,

 

are there any options to replace the spider gears? I had an old diff that didn’t make the cut when I moved house because there was visible wear on the faces (excessively polished faces with a visible step) and there seemed to be some wear in the shafts that they run on. Should I regret sending that for scrap?

 

I could have always applied the Lincoln diff locker. 

 

 

Edited January 21, 2019 by Simeon

[nbristow01]

Thanks Toby, I have an early diff so may search for a standard later 02.  The idea of 3.91 with a 4 speed for a 6 hour drive is painful.  May only be 300 RPM difference but that is actually quite a bit when you are in the car for hours.  

Next summer I may look for a good 5 speed and do a swap like my last car.  My goal now is get this can completed and to MidAmerica...5 speed swap can wait

[TobyB]

When I was in the diff market, the E21's were so plentiful that I never looked for solutions to 

the spider gears, Simeon.  And I was welding them, too, so it just wasn't an issue.

 

Bkuz rasekar.

 

t