Diff calculus

[Duran]

Hi guys
I am about to embark on a rebuild of my 8 3/4 and am already confused

I have a big feeling I have already bought the wrong gear so please can some one set me straight

I ve purchased a new Posi track to suit 30 spline axle
I ve got a new ring and pinion set for 39 11 teeth 10 spline &
I still don't know what spline my axles are or how to tell (do I just count the number of teeth?)

Sorry guys I m lost here

[AKcharger]

I've always farmed out my diff work, not rocket science but to me money well spent

[John_Kunkel]

I have a big feeling I have already bought the wrong gear so please can some one set me straight

The casting number on the lower left side of the third member tells you what gears you need, last three digits are usually 741, 742 or 489; the difference is the pinion shaft diameter. 39-11 is a 3.55 gear set.

All factory 8 3/4" axles are 30-spline. The yoke spline can be determined by the hex of the yoke nut, 1 1/8" is 10-spline while 1 1/4" is 29-spline.

The FSM is your best guide for the setup procedures.

[Duran]

That does shed some more light for me
And I did check my housing it’s a 489

also just checked my yoke nut which was 1 1/8 so a 10 spline


so i have a feeling i might have assed it and got the right gear as my new gear and spline are for a 10 spline woo hoo

[John_Kunkel]

The 10-spline pinion was available in all three housings, you need to measure the pinion stem diameter to determine if you have the correct gear set. A 489 pinion stem will measure 1 7/8" next to the gear. What's the brand/part number of the gear set?

[Nacho-RT74]

These are the pinion differences between third member case ( aside the spline count ):

http://www.moparts.org/Tech/Archive/axle/1.html

Also 489 uses a conical collapsible spacer for bearings preload