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ID of LSD from Exterior


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There are many discussions and threads here re: LSD and their identification.  What I need is a reasonably reliable and, hopefully simple, way to determine from the exterior if an E21 diff is an LSD.  I likely will have to crawl under the car and will limited access and restricted sight lines.  # of bolts?  Length?  an "S" somewhere?  a blinking neon light indicating "LSD. LSD.  LSD..."?

 

I have never seen an LSD in person and am not entirely sure there is a way to do this w/o removal of the diff

 

Larry

 

 

 

 

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They did use a stencil to paint a white S on them, but that'll be covered up if it is greasy.

 

One thing you can do is to raise the rear end and spin one of the wheels.  If it is an open differential, the opposite wheel will spin the opposite direction.  On an LSD, it is supposed to spin the same direction as the one you are turning.


Tom

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Since it's highly unlikely that a previous owner has removed and repainted a differential, there should be a rather large, white S painted on the side of the diff.  Also, a LSD will have a letter stamped along with the ratio is that's either leading or trailing the ratio (expressed as two numbers, denoting the number of teeth on the two gears).  The letter denotes an LSD

 

Also, if it's an E21 you're checking out, if it (still) has the Recaro sport seats, a trunk lip spoiler and mesh wheels, it's a 320is which by definition will have a LSD--3.64 for a two liter engine (77-79) or 3.90 (80-82).  

 

mike

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The big white “S” doesn’t seem to appear until ca. 1979, possibly later. My 1978 e21 LSD, currently in my ‘76, has the stamped “S” between the number of pinion and ring gear teeth (i.e., “11   S   40”), but never exhibited any evidence of a stenciled “S” — even before rebuilding.

 

My ‘73’s bona fide ‘02 LSD, removed from a rusted-out ‘72 tii, acted like an open differential when I purchased it, because the locking effect had deteriorated to near-zero from wear. So the old “same-direction/reverse-direction” test is not foolproof!

 

If I saw a stamped “S”, as seen on my ‘78 LSD below, I’d be willing to risk that the housing still contained an LSD. But repos of the white stenciled “S” are proliferating, so I’d place little faith in a white stenciled “S” unless the S was really faded and the unit was REALLY filthy and greasy!

 

Regards,

 

Steve

 

 

C84F7F6C-4BFE-4F51-B7FD-BC73D05D8AB8.jpeg

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9 minutes ago, Oldtimerfahrer said:

Mine was stamped, but hid a broken pinion. If you do drain the oil and have a peek inside with a cheap usb camera attached to your mobile phone, you have a better chance....short of taking the rear cover off...which would be the best...

A.

drain the oil????  remove FILL plug and look....no need for any draining.  😎

 

but 99% of the time, the spin the rear wheels test will work.  an LSD has to be REALLY worn out to fail that.

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3 hours ago, M2M3 said:

drain the oil????  remove FILL plug and look....no need for any draining.  😎

 

but 99% of the time, the spin the rear wheels test will work.  an LSD has to be REALLY worn out to fail that.

but a brand new diff will also behave like an LSD due to the minimal play

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27 minutes ago, uai said:

but a brand new diff will also behave like an LSD due to the minimal play

have spun a lot of wheels checking diffs, including new ones.  never seen an open diff act like an LSD.  

 

but have never checked a NEW 2002/320 open diff.....LOL....is there even such a thing?

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10 hours ago, M2M3 said:

have spun a lot of wheels checking diffs, including new ones.  never seen an open diff act like an LSD.  

 

but have never checked a NEW 2002/320 open diff.....LOL....is there even such a thing?

I had a brand new open diff (on the bench, no Oil in it) and the friction of the spidergears was so high that if you turned on one side the other turned the same way.

Also if you have a LSD without preload on the bench you cannot tell what it is by turning by hand.
 

Edited by uai
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There is this old saying "seeing is believing". If someone is trying to sell you a "known, good, not worn out" LSD, then the spin test is the first test. I spent 500 on a "rebuilt" E30 188 case LSD, nice and clean sitting on a bench... only to find that the casing contained media from the bead blasting, the preload was all wrong, the ramp angles were random, only 2 clutches installed where 4 fit...so spent another 300 remediating the work. Either buy cheap or buy the best with reputable receipt for the work. Everything else is a core from my view. I have an empty "S" stamped cases sitting on the shelf, its not an indicator of anything really... 40-50 years on.

I would of course use the stamped case if I were to build another LSD diff though... 🙂 

Andrew

Edited by Oldtimerfahrer
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