8-3/4 - 489 - 3.23:1 spline count...

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7milesout

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I've been searching and reading posts and links trying to find out if there is a definitive way to determine the axle spline count at the differential / yoke on the Scamp. I'm not familiar with this hardware whatsoever.

But what I'm reading on the Moparts Tech page is that the spline count is either 29 or 10. Most likely I'm guess it to be 29. What I'm trying to figure out is ... I'm wanting to buy an Eaton Detroit TrueTrac, and it seems to state it has either 30 splines or 33 splines. So ... I'm either stuck, or I just really don't know what the heck I'm looking at.

My main question is: Without taking stuff apart and counting the splines, is there a way I can tell if it is 10 or 29 spline? I've heard the bolt head size is how to distinguish it, but I don't know which size means what.

And also, what's up with the Eaton Detroit TrueTrac? Is there a 29 spline option (I'll keep chasing it on that end too). Or am I going to have to convert something to get to a 30 spline?

There's always more to learn, seems like I never know anything.


7milesout
 
You are mingling the pinion spline count with the axle spline count. The axle spline count is what you want. It is originally 30.
Just tell them you need a unit for 30 spline axles.
 
Ahhh, ok. Thanks. Sorry, I'm having to try to imagine all this, and look at schematics doing google searches.

Am I to guess that when using a new differential, I would use the original pinion gear? So that's why the driveshaft spline count doesn't matter (due to not changing that)?
 
You don't need to remove the pinion unless you are replacing the bearing, the seal or replacing the crush sleave.
Personally I would replace all the bearing and install a crush sleave eliminater while I was that far apart. You will be reusing your ring gear.
 
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Excellent. What would I do without you smart guys? I've been (sort of) fretting on what gear ratio I was going to go back with. All along I've been wanting to just run the same gear ratio. Heck, if I'm to understand, I don't even have to pull the pinion, and can reuse the ring gear. That makes things easy (or ... easier). I'll look up what this crush sleeve eliminator is. The open diff (or worn out LSD) works really well as is (aside from allowing one tire to break traction while the car barely accelerates), and I'm a believer in, "if it ain't broke, don't fix it." But if the eliminator could "eliminate" future issues, I would probably want to go that route.

Will look at Dr. Diff directly. Thanks for the valuable info!


7milesout
 
Yes. The eliminator I was talking about is for the 489 housing but you can get them for the 9 1/4 also.
 
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Guys - Sorry to dredge this up again. So, 2 things:

  1. You all agree 30 spline count on the axles are correct.
  2. And I can reuse my ring and pinion gears (assuming they are all good, and they work just fine now).
Thanks for you help. It's time for me to buy the TrueTrac.


7milesout
 
7milesout said:
Guys - Sorry to dredge this up again. So, 2 things:

  1. You all agree 30 spline count on the axles are correct.
  2. And I can reuse my ring and pinion gears (assuming they are all good, and they work just fine now).
Thanks for you help. It's time for me to buy the TrueTrac.


7milesout
Click to expand...


1. Yes
2. Yes
 
7milesout said:
Excellent. What would I do without you smart guys? I've been (sort of) fretting on what gear ratio I was going to go back with. All along I've been wanting to just run the same gear ratio. Heck, if I'm to understand, I don't even have to pull the pinion, and can reuse the ring gear. That makes things easy (or ... easier). I'll look up what this crush sleeve eliminator is. The open diff (or worn out LSD) works really well as is (aside from allowing one tire to break traction while the car barely accelerates), and I'm a believer in, "if it ain't broke, don't fix it." But if the eliminator could "eliminate" future issues, I would probably want to go that route.

Will look at Dr. Diff directly. Thanks for the valuable info!


7milesout
Click to expand...

If you have a sure grip, it will be the cone style, easy to rebuild, there is a sticky on here to show you how to do it.
Mill a little of the cones, and shim the rear, almost no dollars spent, if you know someone will a mill.
 
Even if I have a sure grip, I'm going back with a TrueTrac. I'm guessing it's just an open diff. I'll put it up for sale cheap after I remove it. Nothing wrong with it (unless it's a worn out LSD).

I have a desire to do this work all by myself. Does anyone know where I can find step-by-step instructions, for the 8-3/4, 489 diff? I'm good at mechanical work, and decent at following instructions.


7milesout
 
A Mopar service manual dated from before 1975 will have the specs for the 8 3/4.
If you only pull the ring gear out and not the pinion then you only have to install the ring gear on the true track. Then set it in the housing, set the backlash and lock the adjusters. Don't forget you'll need 2 new bearing and races for the true track.
 
The way to tell if it is a 10 or 29 spline pinion is by the nut size that holds the yoke on. If the nut takes a 1-1/16" socket it's a 10 spline. If it takes a 1-1/8" or 1-3/16 socket it's a 29 spline pinion. BTW: not all 489 cases had a cone type sure grip. The early ones (late 68, early 69) had clutch type SG's. Probably due to Mopar wanting to use up excess inventory
 
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