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LSD or open diff ?


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Some time ago I bought a 3.89 KC diff from an ebay auction simply because I won it with a low bid of £40. It was clean and painted and reputedly rebuilt in the distant past but had never been used. I squirreled it away as I had no immediate use for it.



However, I came across it just recently while clearing out my garage and discovered that when the drive flange is turned the drive shaft flanges both rotate in the same direction and if I hold one in an attempt stop it turning, no amount of force will prevent both turning in the same direction !



The diff is empty of oil and is quite firm to turn offering more resistance than I expected but smooth and not lumpy 



I'm certainly no expert but it appears to behave like a limited slip diff. I can't believe I've been lucky enough to actually have acquired an LSD and before I crack it open to peep inside was wondering what possible faults in an open diff could create the behaviour I have observed, as I feel it's more likely to be a knackered standard diff knowing my luck.



Any comments from those in the know would be appreciated.




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I think that it is unlikely to be an LSD.  It is more likely that it has a bit of partial-minor-seizure that will free off with a bit more torque applied.  If it were an LSD it would still behave like a normal diff at low-speed (I believe); the two output shafts can still rotate at different speeds.  The maximum difference between the two shafts is the aspect that is "limited".  This is probably better explained by someone with a racing licence -  over to you John D....



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Just applied "a bit more torque" and did manage to turn one drive shaft with the other locked but it really doesn't want to.


Guess I'm going to have to have a look inside either way, is it simply a matter of removing the output shafts and splitting the casing?

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I have a viscous LSD in my car (subaru diff) and that can be turned opposite by hand but with significant resistance.


Worth having a look inside, you are correct about removing quarter shafts, the split the case. Reassemble with a decent sealant (always works for me!) and don't lose the tapered bolts holding the case together.

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Mystery solved, 


Not an LSD but shiny and clean and obviously rebuilt with engineers blue still apparent so not a bad buy for £40 perhaps. I'll give it a rinse with clean oil and see if it frees up at all as it's still quite stiff, not sure it should be.


Thanks guys.


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There should be pre load on the pinion and diff case bearings , that can make turning by hand a firm

dead feeling , the planets and sun gears dont have a pre load between them but if the tooth backlash has been set to a minimum then working a output shaft again is not an easy spin,


diffs transmit a lot of energy you dont want slack anything the pre loads are there to maintain all the mounting distance settings , under drive , coast and overun loads or you get a symphony of whines



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I'm no expert on setting up a differential, I've never done it, but in theory I'd say that the crown gear isn't set correctly to the pinion.

The drive surface is very much to the inner end of the crown teeth, when it should be central.


If those are that tight, then the sun/planet gears may be the same, which explains the LSD-like behaviour.



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Sorry for the slight drift here, but Pete you talk of a whine...... I get this off my diff but at high speed (over 70 mph). Does this mean that my diff  has too much slack? or on its way to being so? Can it be cured?





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to get the + _ postions you need to run the set on a Gleason tester, probably all been scrapped now,  this gives the mount height best run position error and should be etched on the end of the pinion

the WSM gives a good clue about the contact postions and what you need to change to correct it

theres the mounting distance of the projected cone from the CW centre line and the backlash which needs a DTI perpendicular to the tooth to measure the CW lash or you ramp the measure up  due to the tooth angle 


running just up from the toe and not as as far as central is to me about right, under load the mark will move up the tooth.

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over 70  naughty !!!


whine is often bearings rather than mesh  the tail pinion is the first to suffer from  low oil starvation the inner goes with age as this takes all the load thrust,  as they wear the pinion mesh will change 


if its quite overrun but howls on drive , its likely you need new pinion bearings ,  some change in the mounting height would also 

be a thought 


if noisy on over run i would remove a shim from behind the outer brg, and re test


on drive  ..not much you can DIY


without some equipment or good luck .



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nowt t, worry aboot

in 100-200 miles it,ll bed in, free up


BUTT, I bet ye tried t,turn it frae the half shafts

it,ll be hard as gears same size

if  it done frae prop end, then its a small gear, driv,n a bigg gear, hence easier


at least some ones bunged in new thrusts


drain  yer oil in 200 odd miles, it,ll get rid of the bronze stuff that noo int oil



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It seems OK,ish,....IF the thing es been tried under load, and no just spun owa

under load, loading the CW,wid a bigg prybar, or wack,n a chisel between CW,case


as the tooth contact can move alott, like some times 1/4 inch

all depends as weel on hoo much pree load ye got on side an pin bearings


as too loose, then pattern changes all owa

Its a used diff, and they div,nt always go according t,plan !!!!



Look at the tooth contacts on drive, not all teeth at the same

this is common whenst no under load


the coast side markings are a worry t,me, farr too far doon

which indicates Pins set too low


blue it up, really load it up, an turn it owa, get a marra t,help ye if cant hold an turn at same time

its a 1 1/8th socket, an use a decent sized breaker bar,

Div,nt  jerk it, just turn it 

NOTE, if its a Nylok nut onit, its a collapse spacer,  TOOO much turning force CAN tighten the spacer doon even moer

if it is, be care full, DOT mark the nut,Pin shaft so ye no  if its moved


turm it owa a few times, like 20 turns t,get a really good pattern

then see if the pattern is same as whenst it were on no load,


It should be markedly different

IF the pattern on Drive goes UP the tooth, no where near bottom, an running oft top, then  this confirms pins too low

and needs shims added under either Pin heed, or pin bearing, wid corresponding shims under spacer


can also alter the in / oot pattern too.


bung some piks up after ye done it so I can see,



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Thanks M,


That's brilliant, I think. I'll give it a go.


Not done this before but happy to learn, might take a short while as the kitchen is down to the plaster presently, bear with me.


Only relatively recently rebuilt my first gearbox, (stayed well clear of the overdrive which was functioning perfectly) under the guidance of the late, great Clive Manvers, but diffs are a dark art to me at the moment. Still, I have a book and a set of tools and advice from the forum, what can go wrong ?


Thanks again,




PS  Just fell at the first fence, quick look on the interweb for engineers blue showed up my ignorance, is it a paste ? I was expecting a fluid like copper sulphate solution, or am I showing my age ?

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