Off the shelf 3rd member that drops into our cars?

Sheriff41 · May 15, 2023

Spike Morelli said:
On my car, as it came with A/C, and an FMX, the 3.5:1 ratio was never available for that particular combination, and so, a simple nylon drive gear swap at the trans wasn't in the cards for me. My local Speedo Shop had a unit that goes inline on the speedo cable, called a "ratio box", a compact little piece that they can load numerous combinations of gearing to make the stock speedo accurate. Just mentioning it as an option, for some may not be aware that exists. As for the advising to let a rear end shop do the work, they have the parts, and the experience. Obviously, there are those on the boards who are capable of doing the job, but they aren't ,by and large, the ones posting for advise. I assume anyone inquiring here has little to no experience in differential overhauling, ( include me in that group ), there's other things I'm good at, at I'm OK with that.

Good advice, Spike.

mrgmhale · May 15, 2023

jakosaur said:
I actually just did a rebuild of my third member over the weekend. It went fairly smoothly with only one new specialty tool (I am a quality engineer so I already had all of the precision measuring tools handy. Those probably cost a lot). All I needed was a $30 bearing puller from Harbor freight, and a full day of work. Now I have the 3.50 ratio installed for the AOD swap I’m doing. If you’re slow and methodical in your work, it really is easy but time-consuming, but will also save thousands of $ over buying a drop-in. I think it cost me about $300 including tools. You could also source your own parts and send it to a shop for much less than buying a drop-in. Just my $0.02.

The prior owner of our 73 Mach 1 had already swapped the original 2.79:1 open differential gear set with 3.5:1 TractionLok gear set. About 2 years after we acquired the Mach 1 the C4 began to slip significantly with the 1-2 upshifts at WOT, whereas previously it had a firm shift and we were barking some serious rubber on those WOT upshifts. I decided to research doing an AOD transplant, then went for it. Now we have the best of both worlds, a nice rear axle ration that recaptured the lower speed performance of the Mach 1 and its street/strip rebuilt engine (also prior owner), plus the lower RPM at highway speeds with the AOD. We also gained a higher top end potential as the engine has plenty enough torue to overcome the air resistance we incur at higher speeds.

I am very happy with the AOD, but I do believe that were I to do it over I would have opted to get the 4R70W, or even an AOD-E, despite needing to get an external electronic controller to manage its shifting. With the AOD I have set, and reset the Throttle Valve Pressure three times so far. Each time I have been nudging the PSI up by PSI while chasing a higher road speed upshift point for the 1-2 and 2-3 upshifts. The higher TV PSI approach has worked, and I have no problem running aa little higher than factory specs for the PSI (running too low will end up causing a fried tranny). And now another enthusiast has asked me to video record how I am attaching the oil pressure gauge to the AOD's test port (which I plan to do today, along with another PSI reset).

Connecting the oil pressure gauge to the AOD's line pressure test port may sound like it is easy to do. But, the space I have to work in, after unbolting the exhaust pipes from the header collectors, is quite limited. It is hard to get my hands, tools, camera, and pressure gauge pipe adapters up where they need to be in order for me to adjust the Throttle Valve pressure setting. Every time I get up there I find myself wishing I had gone with the 4R70W, despite a few hundred $ more, as with the electronic controller I could have all those adjustments from the controller itself. Also, the 4R70W is considered to be stronger tranny than the AOD or AOD-E, although the AOD is considered to be able to handle up to 650 HO (our engine has a dyno output of 360 HP, a moderate street/strip build).

I do have some documentation re: our parts and vendors listing I am happy to prove you with as you ponder the AOD option (a smart move in my opinion). I also video recorded the entire swap. The YouTube video links are contained on one of the documents I put together. The files are attached, and also on my Google Drive at:

https://drive.google.com/drive/folders/1-lnrDwhEKchMgNDYtRrIBkbNYjtrt3C9?usp=share_link

giantpune · May 15, 2023

And while we're on the subject, do you guys have any recommendations for which axle bearings and seals to go with? I haven't done enough of them to know which ones are good and which ones are cheap junk.

Sheriff41 · May 15, 2023

I used Timken bearings and National seals from Rock Auto. I think most bearings are sourced from offshore nowadays.

pedstagn71 · May 16, 2023

I also went with quick performance. I got one with 389 gears and another with 325 gears. Both have Detroit locker. Install was direct bolt in with no issues at all. I highly recommend them

jakosaur · May 16, 2023

Sheriff41 said:
I used Timken bearings and National seals from Rock Auto. I think most bearings are sourced from offshore nowadays.

I second this. Timken and National are OEM, still made in USA

jakosaur · May 16, 2023

kulfurs said:
I did a QP swap about 3 months ago. Works great bolted right in. 3.50 gears posi. Using a AOD trans

How’s the 3.50 with the AOD? That’s what I’m working on building now

giantpune · May 16, 2023

Sheriff41 said:
I used Timken bearings and National seals from Rock Auto. I think most bearings are sourced from offshore nowadays.

Do you happen to have the part numbers? I get lost trying to navigate rockauto's website.

Sheriff41 · May 16, 2023

giantpune said:
Do you happen to have the part numbers? I get lost trying to navigate rockauto's website.

Bearings: TIMKEN RW207CCRA
Seals: NATIONAL 9569S

Hemikiller · May 16, 2023

giantpune said:
Do you happen to have the part numbers? I get lost trying to navigate rockauto's website.

Wheel bearings and seals are under the Brake & Wheel Hub category. I always go to Drivetrain first, swear a bit, and then get on the right track.

jakosaur said:
How’s the 3.50 with the AOD? That’s what I’m working on building now

It's a great gearset with the AOD. This is in my 71 XR7 with 245/60R15 tires.

Paul M · May 16, 2023

giantpune said:
I have a '73 that came with the 2.75 open diff. It looks like its about due for rebuild or replacement. I would like to go to something around a 3.25:1 limited slip.
When shopping around for a complete 3rd member, there are variables like axle spline count and which bearing caps the yoke is for. I know some 3rd member housings and caps are bigger than others and may not just plop right in.

Do you guys have any suggestions for a 3rd member that is known to just bolt right into the 73 mustang and work without any surprises?

View attachment 76729

View attachment 76730

giantpune · May 16, 2023

Sheriff41 said:
Bearings: TIMKEN RW207CCRA
Seals: NATIONAL 9569S

Are those part numbers fairy interchangeable? Rockauto only shows one "NATIONAL 9569S" in stock. Summit lists a "Timken Wheel Seals 9569S". And strangely enough, autozone lists a "National Bearing RW-207-CCRA". Like somehow national and timken are the same company?

And I reckon are there any other parts you guys would be changing at the same time while I have the rear axle turn apart? I had the universal joint on my list.

EDIT -
Found this in google. I guess the timken and the national are the same.

Paul M · May 16, 2023

Yep, I'm considering a swap out myself. The open rear does nothing in the performance department. The Detroit locker is too much and will not corner well for street use. The Track loc is a stock friction type (posi) buying used will most likely be worn out & need rebuilt. New is fine & the cheapest way to go posi till you wear it out and have to rebuild it. Then there's Truetrac - Helical-gear limited-slip (worm differential) is the modern replacement for the classic clutch-type posi. Under normal light throttle driving conditions, a Truetrac operates much like a standard open differential, allowing the rear wheels to rotate at slightly different speeds for smooth cornering. However, when accelerating (torque applied), the six helical-gears inside the unit smoothly and quietly engage, applying torque to both rear wheels equally, maximizing traction (think two equal length black stripes). Unlike a traditional clutch-type posi, a Truetrac requires no special oil additive or maintenance. With silky-smooth positive operation and no components to wear out, the Truetrac maintains day-one performance for the life of the unit. I'm most likely going with a Truetrac helical gear type. my problem is I do have over 400hp and a tranny built to take it. I also have 28 spline axles. I'm considering using them until I break one and replacing them with Strange or Mosier 28 spline axles if they break. I'm at the point to where I change out the axles w/ 31 spline or see how much abuse 28's can handle. I'm supposedly ok at 400hp & 450/500 hp is pushing it. I also have the press and dial indicator and measuring tools for the job, however, if you price the individual components and build it yourself it's more expensive than buying the whole unit. I looked into the cost and the shipping of the various parts added up making it more expensive. I have a local transmission supply shop that can get me any parts & spacers I'd need.

jpaz · May 16, 2023

I have the Truetrac with 3.50 gears. I really like it, but it does make a bit of a whining sound, which is normal.

Sheriff41 · May 16, 2023

jpaz said:
I have the Truetrac with 3.50 gears. I really like it, but it does make a bit of a whining sound, which is normal.

I hadn't heard the Truetracs whine. I'll have to listen closer to mine. I have Truetrac with 3.89:1 gears.

Paul M · May 16, 2023

Sheriff41 said:
I hadn't heard the Truetracs whine. I'll have to listen closer to mine. I have Truetrac with 3.89:1 gears.

yep that must rip with 389's this cam n converter requires some gear. I'm debating 3:70 - 3:89 for this build. (C6 w3200k) I hope I don't continue driving it like a teenager though.

Paul M · May 16, 2023

jpaz said:
I have the Truetrac with 3.50 gears. I really like it, but it does make a bit of a whining sound, which is normal.

ha I won't be able to hear it over the electric fuel pump.

Sheriff41 · May 17, 2023

Paul M said:
yep that must rip with 389's this cam n converter requires some gear. I'm debating 3:70 - 3:89 for this build. (C6 w3200k) I hope I don't continue driving it like a teenager though.

Depending on your setup and objective I'd recommend the 3.70s over the 3.89s. Better all-around ratio, IMHO.

Ron Tanzi · May 17, 2023

My have prices for a 9" center section risen. I bought a complete rebuilt open 9" 28 spline four pinion with a my original 2.75 ratio from Currie in 1997 for $475 with shipping. It is still in the car. I have my original unit with the "intent" of installing a 3.25 ratio. I am still undecided on what locking system I will use. I ruled out a Detroit locker ages ago. I am glad I waited considering how many new options there are now compared to 1997.

giantpune · May 17, 2023

Ron Tanzi said:
My have prices for a 9" center section risen. I bought a complete rebuilt open 9" 28 spline four pinion with a my original 2.75 ratio from Currie in 1997 for $475 with shipping.

$475 in 1997 dollars is $897 in 2023 dollars. Thats not too far off from some of the prices I'm seeing as I'm shopping around. Even including some limited slip ones.

Off the shelf 3rd member that drops into our cars?

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