Just bought a 8 3/4 have a few questions?

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soaringcowman

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Just bought a complete 8 3/4 rearend for my 71 duster. I just picked it and a drive shaft with a few random mopar parts up for $200. The guys told me it wasn't a posi track rear end. But when i got home and spun the axle both sides turn the same directions. And the carrier says it's a 489. But my main question is whats the gearing? I spin the axle 1 complete turn and the yoke on the carrier spins 3 1/4 turns. What would this make it?
 
The guy was right, it's not a posi trac. They were installed in GM cars. Mopars had limited slip differentials. The real test is to try and hold one axle while trying to turn the other. If the pinion turns or the axle turns easily it's not a limited slip.

BTW 3.23 sounds correct.
 
Guitar Jones said:
The guy was right, it's not a posi trac. They were installed in GM cars. Mopars had limited slip differentials. The real test is to try and hold one axle while trying to turn the other. If the pinion turns or the axle turns easily it's not a limited slip.

BTW 3.23 sounds correct.
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A posi-trac is GM's name for a limited slip. A sure grip is Chrylser's name for a limited slip. Both Chevy's and Chrysler's could be had with limited slips. Careful with your correction.
 
I believe the point GJ was trying to make is that it is not a posi "type" diff. Posi, Sure-grip, and more recently "no-spin" are just the OEM nicknames given to a friction-joined type diff. The Dana or manufacturer name for their 60s-70s era clutch type posi's were Dana Powerlocks, for example. The later cone type models from Auburn were different types of the same technology, I just forget what Chrysler called them.

Semantics, man. No need to get picky about it.
 
Well i did like GJ said, i held on side still and the other side turned and so did the pinion. So i'm kinda confused, whats this mean i have now? and what did my 7 1/4 have cuz when you spun it with both tires lifted the tires turned opposite directions?
 
The only thing that’s spinning is my head.
Have you ever seen how a differential works? It is a thing to marvel at.
I’m sitting here trying to visualize how the “spider gears” of an “open” will make the other wheel turning the opposite direction if the pinion is held.
I will guarantee that your 7 ¼ was open.
That sort of changes things when you are doing this.
In other words is the pinion being held when you do these things to any given rear end?
Things get confusing.
:toothy10:
 
adriver said:
What you paid would “make it” a screaming good deal , no matter what the 489 is.
Glad you scored it.
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That's the part of his original post that stood out for me. I was at Mopar Nat'ls last month and $200 is a fantastic price for it. Wish I could find one w/driveshaft and complete with center section for that price!
 
yeah my head is spinnin also, i'm confused on what i've got lol. i just need to get some drums, pad, and wheel clyinders and it will going up in the car. I also need the steel break line that runs across the top, on of the ends is snapped off were it screws into the cylinder. Just couldn't pass any of it up for the price!
 
if its a sure grip, you shouldnt be able to spin one tire with the other one locked in place. think of a sure grip as a clutch disc clamped tight between each tire. if you turn one tire and the other side turned the OPPOSITE way then its not a sure grip..........but a 489 is one hell of a great deal for $200
 
the 7 1/4 spun the opposite direction now my new 8 3/4 with the 489 casing spins the same direction when spun, but Guitar Jones mentioned to hold one axle still and then to try spinning the other axle. And when i did so i was able to spin the other axle not being held and the pinion also spun while i spun the nonheld axle.
 
soaringcowman said:
...Guitar Jones mentioned to hold one axle still and then to try spinning the other axle. And when i did so i was able to spin the other axle not being held and the pinion also spun while i spun the nonheld axle.
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He wouldn't be able to do that if the spiders were welded.

It coudl just be a worn out sure-grip.
 
If you hold the pinion still and spin one axle the other axle will spin the opposite direction if it is an "open" carrier.

Chuck
 
doug 371 said:
take it apart and make sure its not welded
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You know, if you don’t need this tomorrow, exploratory surgery is a good idea anyway.

If it is old and well used or if someone else have had their hands in it.
I always like to look at things before I use them.
I had a 489 pinion nut come loose on me a long way from home.
I’m sure someone had messed with it.
I hang the 8 ¾ chunks in a tree by the yolk and hose it out from underneath with mineral sprits then put fresh oil in the rear when I take the rear apart for cleaning.
 
It's an open rear or a very worn out limited slip. It's not welded either.

I think your seller correctly represented the rear to you.
 
i hate saying this cuz i don't wanna sound dumb, but when you say open rear end, what does that exactly mean?
 
"open" means non-locking differential. In an open differential power is sent to the axle with the least amout of resistance (traction). On a "locking differential" the power is sent to the axle with most amount of resistance (traction) or at very least the same amount of power to both axles. Asking questions is one way how people learn so don't fell bad about asking.


Chuck
 
So now what i'm wanting is "sure-grip". Which let me know if i'm correct..this applies power to both wheels in the straight, which lets me lay down 2 nice thick black marks :). but through the turns it will dispurse power more to one tire then the other to allow you to turn easier and with out both tires turning at the same speed?
 
soaringcowman said:
So now what i'm wanting is "sure-grip". Which let me know if i'm correct..this applies power to both wheels in the straight, which lets me lay down 2 nice thick black marks :). but through the turns it will dispurse power more to one tire then the other to allow you to turn easier and with out both tires turning at the same speed?
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you pretty much have it. in a turn the rear will let the clutches "slip" if traction on both sides is equal. however, if you are in rain or snow and traction is not good, both tires may spin at the same speed causing loss of traction and control.
 
now my other question is that my 8 3/4 didn't come with drums or wheel cylinders. but now if my 7 1/4 has the same size drums as what the 8 3/4 has would the parts be transferable from back plating to back plating?
 
sweatybetty said:
y...however, if you are in rain or snow and traction is not good, both tires may spin at the same speed causing loss of traction and control.
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Good advice, for sure. Many a nice Mopar has ended up in the weeds, wrapped around a pole or worse after an inexperienced driver with a sure-grip lost it.

The Richmond Lockright in my Duster is even worse, scary sometimes. I have to be really careful in the first three gears, even on dry pavement with the 3.91s. But straight ahead is real fun!!
 
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