3.23 8.75 LSD, OK, or...?

[70Hardtop]

I just put a 3.23 SureGrip (741 case) centre into my 73 Rallye 340 Challenger. This centre had previously had new carrier bearings fitted and the unit bench checked for normal operation.

With the car on stands and no drive shaft fitted, I noticed some things, can anybody who knows these 8.75 centres tell me if these characteristics are correct please?

With just one axle in (RH side), the axle turns easily with a loud noise coming from inside the housing, like a clicking sound. Nothing else turns. I expected the input part (yoke) to turn also.

With both axles fitted, when one axle is turned, the other axle turns the opposite way. (I thought that Limited Slip diffs should turn the same way, I thought only open centres (non-LSD) turned in opposite directions? Also the yoke does not turn.

Now when the yoke is turned (it is very stiff, it needs a lever to be able to turn it), both axles turn the same way, but can be stopped with your hand, even though the yoke is turning. Is this the LSD clutches or cones working?

Are all these characteristics normal?

Thanks

 

[RUNCHARGER]

In a word no. Hard to diagnose from here but what you are describing is not normal. The clicking sound isn't right and you should be able to turn the input yoke by hand. If the LSD is working correctly one side will not turn opposite of the other either.

Sheldon

 

[aussiemark]

I agree with Sheldon and it sounds like the pinion bearing may have too much preload.

 

[70Hardtop]

I agree with Sheldon and it sounds like the pinion bearing may have too much preload.

Thanks for the answers. I have heard about preload and was wondering if that was causing it to be so stiff. Should I be able to turn the yoke relatively easy by hand, even with new bearings? Also, is it easy to re-adjjust the preload lighter? I have the proper service manual for this car so I will read up on it and see if I can do it myself. I have a torque wrench.

Thanks

 

[70Hardtop]

In a word no. Hard to diagnose from here but what you are describing is not normal. The clicking sound isn't right and you should be able to turn the input yoke by hand. If the LSD is working correctly one side will not turn opposite of the other either.

Sheldon

Thanks for the reply Sheldon. Looks like I will have to pull it out again (*&#@). Someone also told me that they think that 1966 and earlier 741's wheels spun opposite when in the air. But that doesn't make sense to me. I'm not even sure if this unit has clutches or cones. I thought the SureGrip had cones and the Trac-Loc had the clutches. Looks like I will have to do some reading and try and fix it myself with the service manual.

 

[DetMatt1]

Ma Mopar never used the term posi.

 

[70Hardtop]

Ma Mopar never used the term posi.

Oh ok, sorry for the error, I better change that GM name to Trac-Loc. But your post is only half a post, why didn't you expand on it and say it yourself? Would have been more helpful.

 

[moparleo]

Welcome aboard. Here is the definitive article on 8 3/4" differential types:
Mopar Rear Ends / Differentials / Etc

 

[70Hardtop]

Here is the definitive article on 8 3/4" differential types:
Mopar Rear Ends / Differentials / Etc

Thank you MoparLeo ! That's very helpful. I was just trying to find some info, but only getting the company websites (eg Auburn, Spicer)

I will read it soon.

I can't believe I spent $500 on this diff ($200 to buy it, $100 for the carrier bearings and $200 to get them fitted) and it is not behaving correctly. Grrr.

 

[DetMatt1]

Sorry about that, I ended up getting a call and had to rush out the door. I'm actually surprised to see that I hit the post reply button. Good article Leo.

 

[mmissile]

Hey Matt....ma mopar has been dead for a long time.

 

[DetMatt1]

God rest her ever loving soul!

 

[aussiemark]

Thanks for the answers. I have heard about preload and was wondering if that was causing it to be so stiff. Should I be able to turn the yoke relatively easy by hand, even with new bearings? Also, is it easy to re-adjjust the preload lighter? I have the proper service manual for this car so I will read up on it and see if I can do it myself. I have a torque wrench.

Thanks

If only the side bearings were replaced the pinion pre load shouldn't need to be disturbed, perhaps the crown wheel and pinion are meshed too tightly together this should be easy to correct and because you haven't used it no damage would have caused. With the pinion pre load some diffs use shims and some use a crush tube, if it is a shim type you can just change to a different thickness shim but the crush tube can only be tightened and crushed some more if it has been crushed too far you need a new one and start again you can't just back it off.

 

[70Hardtop]

Welcome aboard. Here is the definitive article on 8 3/4" differential types:
Mopar Rear Ends / Differentials / Etc

MoparLeo, I read that article and sorry to say , was disappointed, especially with the way it is written - sorry, but to me that guy sounds like a jerk and should not write. What he has written is opinionated and mostly about his stuff, not technically written, very unprofessional (talking about trying to blow up a diff), not the least that he can't even spell. It is not what I would call a definitive article and is sparse on much information needed - I found what could definitely be called "a definitive article" here: Chrysler 8-3/4" Axle Guide

It has a wealth of technical information and everything you may ever need to know right down to centrelines, shim sizes and bodyline sources

 

[70Hardtop]

If only the side bearings were replaced the pinion pre load shouldn't need to be disturbed, perhaps the crown wheel and pinion are meshed too tightly together this should be easy to correct and because you haven't used it no damage would have caused. With the pinion pre load some diffs use shims and some use a crush tube, if it is a shim type you can just change to a different thickness shim but the crush tube can only be tightened and crushed some more if it has been crushed too far you need a new one and start again you can't just back it off.

Thanks Mark, I hope you are right. But it was a number of years when it was done, and I just can't recall exactly what he did. However, according to this definitive information here: Chrysler 8-3/4" Axle Guide, the 741 and 742 cases used shims for preload (only the 489 cases used a crush tube and used shims for setting pinion depth). I ordered a maintenance kit from Mancini Racing and gave it to the fellow at the time, that included most replaceable parts for all the 8.75 carriers (crownwheel bolts, bearings and cups, shims and crush tube, pinion seal, etc). I think he just did it up a little too tight. The other day I pulled out a 489 case, 3.55 Sure-Grip and that can be easily turned by hand on the bench, no effort at all. However it is full of oil. I rang a diff place here locally and he reckoned it may be ok and advised me to fill the oil as per normal and just keep working it or running it on stands and see what happens. He said with no oil and having been sitting for a while dry, it may just have stiffened up. So that is my first plan of action.

 

[aussiemark]

When you set the preload on the pinion you use a spring scale to measure how much force is needed to turn the pinion by putting the hook in a flange hole put the spring gauge in a position so it can turn the flange not just pull on it if it measures too low on the scale the preload must be tightened and if the reading is too high on the scale the preload is too tight but you really should be able to turn it relatively easily by hand with lightly lubricated bearings.

 

[70Hardtop]

When you set the preload on the pinion you use a spring scale to measure how much force is needed to turn the pinion by putting the hook in a flange hole put the spring gauge in a position so it can turn the flange not just pull on it if it measures too low on the scale the preload must be tightened and if the reading is too high on the scale the preload is too tight but you really should be able to turn it relatively easily by hand with lightly lubricated bearings.

Thanks for the tips. Re turning force, yeah that's what I reckon too. But according to another fellow I have spoken to, he had his BorgWarner diff done and that was very stiff on the bench (intended like that apparently) and ran quite hot for a while, only loosened up after about 100km. He said the guy who did it was an expert on diffs. I will run it up on jack stands with oil in it for a while and see what happens. The main thing I am worried about is the lack of LSD action, I am hoping that the clutchs are just not sliding properly due to no oil.

 

[70Hardtop]

I'm not sure why the pinion is stiff, I don't think it would be foreign matter, but now with oil all around it, it is a little easier. However it still behaves like an open centre. And no, with it out of the car and no axles, it was still super tight to turn. With both axles bolted in, and turning one axle, the other wheel turns opposite (like an open, non-LSD centre) and the yoke does not turn. (unlike the BorgWarner LSD in my Valiant, the yoke will turn when you turn one wheel). Also, when another person turns the pinion, either axle can be stopped with one finger pressure! I did an experiment -see pictures. I have another 8.75 (489 case) with a Sure-Grip -but this one is the BorgWarner cone style whereas the faulty 741 is a clutch, Dana style type. I know that one is good. I stuck axles into it on the ground and did the same checks -totally different ! The yoke is easy to turn with two finders. Turning one axle will turn the other axle the same way, and when the pinion is turned, the axles are locked into gear, they cannot be stopped with force.

So it has to come out. My guess is that the clutches are fried, non operational, and that the guy I bought it from (who also did the carrier bearings) may have known this, I think he tightened the preload too much. And here in Australia, these 8.75 centres are not easy to find. Hardly ever see them for sale and to fix this one with a new LSD unit (eg Auburn Gear or Nitro posi) with the install kit and shipping etc is about A$1400-$1600.

 

[RUNCHARGER]

Hmm: It sounds like you might have to go to square one and start at the beginning. A clutch kit for the Sure Grip runs approx $200 and should be available from a diff. supplier that deals in Dana-Spicer (should be available locally). Strip the crown gear/diff off and check your pinion turning torque with a torque wrench. A factory service manual is a great guide to get to the bottom of this. The good news is all the parts you may need should be fairly cheap.

Sheldon

 

[70Hardtop]

Hmm: It sounds like you might have to go to square one and start at the beginning. A clutch kit for the Sure Grip runs approx $200 and should be available from a diff. supplier that deals in Dana-Spicer (should be available locally). Strip the crown gear/diff off and check your pinion turning torque with a torque wrench. A factory service manual is a great guide to get to the bottom of this. The good news is all the parts you may need should be fairly cheap.

Sheldon

Thanks Sheldon, yes I think that is my only viable option really, me not being Richy Rich. My mechanic suggested that also. But I wouldn't get anything locally - even if these guys in my city were used to getting stuff from the States, they would double the price for their own efforts. Mostly though, ordering parts like that would be in the "too hard basket". Better that I shop around and get the parts myself.

While I'm thinking about it, how important is it to put new bolts in for the LSD centre? (the ones that go through the crown wheel).
Or is that only required when changing the ring gear on the crown wheel?

Thanks