Unclear LSD research

Nicad

Nicad

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I have been reading on the forum about identifying an LSD Diff by jacking up the car and spinning the wheels, then note the direction of rotation. Both my out of the car diff and the one in a local junkyard fail this test. One is way too covered in old oil to find any markings as well. I then came across this message from forum member Velocewest


"My '70 2800CS fails the "wheel spinning on jackstands test" too, but it has an LSD and will leave the same twin stripes as in Mr. Petrus' picture. I've yet to find a sideloader LSD that passes the off the ground test, including the recently rebuilt 40% 3.64 in my e12. That car lights up both rear 235/45's with ease, and on track days leaves no doubt the LSD is working. I don't think that's a good test for these older diffs.

Another thing you guys should be looking for -- on the older diffs, the "S" is not always a painted stamp -- on my cars, it's stamped with a metal die stamp, just like the ratio numbers, and it's only about .5" tall."

Does anyone know if this is accurate? Can a regular diff be made LSD with available parts? I think I'd like an LSD in mine and it is in no position to lay a patch at the moment.
 
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Bob,

All I know is that it won't necessarily have a painted S but it will be stamped with a S. A standard diff can have a locking unit installed in it too. I have also thought that raising the rear end and turning one wheel will make the other wheel turn in same direction. I will check this theory out tonight on mine.
 
Hey Thanks Chris. The drum brakes on the car I am looking at are pretty frozen. I used a bar to break them free and noticed the opposite wheel spun backwards when I broke the seized side free rotating it forwards. Or not at all when I spun from the semi free spinning side.
AT any rate, do you think it is worthwhile buying a non LSD diff just to have as a spare? Are good diffs hard to find these days? I guess shipping is hefty on a diff if none are local. .
 
Probably the best $30 (taxes in) I have spent. After spending a good portion of yesterday scouring the wreck, I was running out of time and banged and cut my forehead in my first attempt to free an axle bolt when the wrench slipped. The price I was quoted for the parts I had in mind was $200 and $230 with the diff. Today I was humming and hawing about making another slog to the junkyard. (40 minute drive) to get a part that I didn't think I'd need. I got there a little late and was missing some of the tools I thought I had packed.
The E9 is sitting on top of an Acural TL. All day cars stacked two high are shuttled almost overhead on a forklift Payloader. One fell off not too far from where I am set up yesterday.
I was surprised I could get the rusty axle bolts off with my 3/8' drive socket, but it went smoothly. Loaded in the car I went and bought a can of Gunk and took it to the car wash. The Grime and dirt were very thick. After 15 minutes of scraping with a wire brush and rinsing this is what I have


Home now, walked the dog noticed how sore my back is and then went to get my reading glasses and a flashlight.

Lo and behold ...an S


I am very pumped about this diff. How does this Ratio work with a 4 speed or 5 speed?

This one failed the spin test, so maybe my other one is LSD too. Can't find an S on it though. The spin test is not accurate. My thanks to Velocewest!
 
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Either should work fine with a 3.46 ratio diff.

Used LSD's $1,000? wow.
 
Great reward for effort. I just bought a LSD for $1100 so I reckon you have saved $1070 assuming the diff is okay. At that saving you can have it fully rebuilt and still be ahead.

38 divided by 11 is 3.4545 recurring to be precise. I read somewhere that the engine revs at about 2000rpm at 70mph. Apparently a great ratio for a modded M30. Can't wait to find this out myself with my 3.45 LSD.
 
I've got a 3.45 LSD in the vranedom car. It passes the spin test. If an "S" diff doesn't pass the test, it doesn't mean that it isn't an LSD. It just means that the clutch pack has worn out to the point that it doesn't lock the carrier when cold. Once the diff heats up, tolerances may close up, restoring some degree of lock. However, you are getting close to EOL on that clutch pack. IIRC, stock lock-up is 45%. If the diff fails the spin test, then it is considerably below spec.
 
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Correction: 25% lock, stock. Clutch pack composition has likely changed since 1972, new packs might have greater thermal expansion. Rebuilders may take this into account, with rebuild not passing the traditional spin test when cold, but providing plenty of lock when hot. Anyone with a rebuilt LSD want to try the spin test with a hot diff?
 
40% lock until 4/71, after that point 25%. The lock up units are totally different too.

With the 3,45 rear axle you'll get 62 mph at 3000 rpm in top gear with the stock box and 62 mph at 2500 rpm if you have a 5-speed. The difference is 18,7% to be accurate. The actual speed may differ lightly due to tires of course.
 
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This thread got me wondering....the car passes the time honoured spin test as both tires turn the same direction cold (this I checked when I first got the machine) But I always wondered what ratio I had. So after cleaning off forty years of accumulated grime I found the painted "S" and the case is stamped 11 - 40. This surprised me as the car definitely has more "legs"
than I have balls. (4 speed) Works out to a 3.63. OK. What modern oil does the consensus recommend for the LSD?
 
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