Axle/diff upgrade variables

[Vinnie]

Hey folks,

Currently I have a 9" axle with 2.75 gears, open diff, drum brakes and probably 28 spline axles. I'd like to change to 3.5 gears and an Eaton Detroit Truetrac diff, disc brakes and possibly 31 spline axles. So I've been reading and learning about this stuff for a week now and instead of narrowing down specs I only see more variables:

- Axle offset (for the brakes)
- Axle bearing diameter (for brakes)
- Pinion diameter (for gear upgrade)
- Carrier bearing size (for new diff , third member housing size??)
- Axle bore (for 31 spline axles)

I am not yet ready to take the rear axle apart so I can't measure anything now. Can these variables (particularly the last 3) be determined without taking the assembly apart?

Thanks,

Vincent.

 

[jpaz]

Well I’m no expert but I do have that same Eaton Tru Trac with 3.50 gears. I had a local shop do the change over for me though. I went from an open differential with 3:25 gears to the 3.50 gears and the Eaton and it is awesome! If you do very much freeway driving you might want a 3.25. 
I kept the 28 spline axles, no need for me to change them as I’m not pushing that much hp. I think I’m around 425 at the most, so I’m not worried about them holding up. As far as the drum brakes go, I have front disc with the drums out back and I have no reason to change. They work pretty good and I don’t think it’s worth it to change. But if you have drums all around then I would definitely do the front and maybe back too.

Im sure others will be more help but I thought I would let you know my experience with what I did. Good luck, hope that helps some!

 

[Don C]

To begin, I would see if the axle tag is still on your car, that will save you a lot of time when you want parts. The tag number and the Ford parts manual will tell you a lot, including the number of splines, bearing numbers, and axle shaft lengths.

You can get a lot of information from these:

Ford Axle History and Identification: Ford Differentials (diyford.com)

Custom 9-inch Axles - Currie Enterprises

Ford Rear Axle Assembly Identification - Page 01 - FORDification.com

 

[Kilgon]

+1 with Don.  Also, if you ever have a tag come off / break off make sure you label what it is from and don't throw it away.  They are worth their weight in gold for the information they provide.

 

[Bill73Ragtop]

Well I’m no expert but I do have that same Eaton Tru Trac with 3.50 gears. I had a local shop do the change over for me though. I went from an open differential with 3:25 gears to the 3.50 gears and the Eaton and it is awesome! If you do very much freeway driving you might want a 3.25. 
I kept the 28 spline axles, no need for me to change them as I’m not pushing that much hp. I think I’m around 425 at the most, so I’m not worried about them holding up. As far as the drum brakes go, I have front disc with the drums out back and I have no reason to change. They work pretty good and I don’t think it’s worth it to change. But if you have drums all around then I would definitely do the front and maybe back too.

Im sure others will be more help but I thought I would let you know my experience with what I did. Good luck, hope that helps some!

I'm in agreement with one supplement:  If you have the C4 / C6 3 spd auto, stick with the 3.25.  AOD or better, go with 3.50.

 

[Vinnie]

Alrighty, I found the tag under a thick layer of crap:



WES-T is a 28 spline axle. Manufacture date june 3rd 1973. 2.75 gears, open. 9".

Why I am thinking of going 31 spline is because I am already changing the differential where I have a choice between 28 or 31 for the same price. So why not go 31 spline all the way? At least if a bigger engine is ever put in the rear axle is ready for it.

3.25 might be a good idea, I'll have a look.

So maybe I'll buy the diff and axle now and wait with ordering the bearings and brakes untill I take the axle out. There is one thing though: I read that the 31 spline axle is a bit thicker and in some cases is too big for the axle housing which needs to be drilled out. Can the axle always be drilled out to make the 31 spline axle fit? Or should I wait untill the axle is out?

Cheers,

Vincent.

 

[Don C]

The 31 spline and 28 spline axles use the same wheel bearing for '71 to '73 Mustangs. As long as the axle shafts are direct replacement axles you shouldn't have any problem.

The bearings are D0AZ 1225-A,  1-3/8" ID & 2-13/16" OD.

 

[Vinnie]

So a direct replacement 31 spline axle can be inside an assembly originally for 28 spline axles?

 

[Don C]

Yes, both splines used the same housing, all with tag prefixes of WES, WFB, WFD, & WFU.

 

[Vinnie]

Cool, thanks!

 

[Vinnie]

One more question: Do 28 and 31 spline axles have the same axle offset? When I look at the lengths (for instance this pair: https://www.summitracing.com/parts/msr-a31-71must/make/ford/model/mustang/year/1973) it seems they have the same length and probably also the same offset? That would mean I could measure the offset on my currect axle and order these...

 

[tony-muscle]

Just find an axle with the same offset. Technically they could be custom made to any offset. If i remember correctly our offset is 2.5". I replaced my axles with 31 splines as well when i upgraded to Truetrac and 3.5 gears. I bought my axles from Strange and they had the option of preinstalled bearings that had an o-ring so you dont need the oil seal. Works awesome.

 

[Don C]

You don't need to specify the offset (BS-brake space) unless you are ordering custom axles. Tony is correct about the offset, 2.5".

If you order those axles they are direct replacement, no opportunity to specify offset.

 

[Vinnie]

The reason for bringing up the offset is coz I need it for the upgrade to disc brakes. 2.5 it is then.

 

[jspears]

I like the true trac, since it never locks or unlocks there is no shocking the tires.  I ended up buying the Currie housing package that comes with 31 spline axes. It had 3 inch tubes though so I had to slot my leaf spring plates to take the larger u-bolts.