Hi All,
1976 Oldsmobile Cutlass Supreme, 350/350, pretty much all original. One of the biggest things I'd love to do with it right now is upgrade the differential. Would LOVE to put in a Posi with gearing about 3.5 or maybe a bit higher (currently has about a 2.4 or so, a bit of a dog at the start...). I was quoted by a nearby reputable shop that to do this would cost me upwards of about $2500, as they recommend a complete rebuild of the differential (since everything, including seals, etc. are all original, no leaks yet, though). I've seen the gearing isn't very expensive, nor is the posi, so how hard would it be for me to do the work myself and save some money (I'm mechanically inclined, but have never done something like this before)?

Many thanks for your advice!
james

 

Reach out to member MONZAZ on the forum, he is in that business and can probably give you lots of useful info.

 

Or www.JDRace.com Jim is the Guru of rear ends on this site.

 

Changing out the differential and gears for a posi and new ratio pretty much is a complete rebuild of the rear axle. You'll want to put new bearings on that new pinion and posi unit. You'll also need to replace the pinion seal when you do it. About the only parts that don't get replaced in the process are the outer wheel bearings and seals, and frankly, you really should do them now also. The good news is that your car has a corporate 8.5" axle, so parts pretty much grow on trees. I have no idea what the going rate is for this work, as I do it myself.

 

If you visit the JD Race website you can see I have help the hobbiest set up ring and pinions....If you are in a rush then you will need to pay someone to do it.
I can help but it has to be on my re-answer time frame.

I do not mind helping and have helped many from a far...
You have to be willing to assemble and disassemble many times and give info to help you set-up your differential.

Many have had great fulfilling success .... It is up to you and how much detail you do as to how well the rear will turn out.

Jim
JD Race

 

Apology for the late response, and thank you all for the great comments. While I would enjoy the opportunity to do (and learn) the project, I don't think my skills are at that level just yet. I'll save up the money and have it done professionally.

james

 

It's really not difficult, just time consuming. I'm working on mine, when I have weekend time available, LOL, with Jim's help from afar. No big deal, really. There are scores of YT videos on the process, some better than others. My current speed bump is I mis-measured my new pinion depth and got a bit too much shim distance installed. My pattern shows that the pinion is too deep into the ring gear. Because of this I have to take it back out and decrease oh-so-slightly my pinion depth. Of course this means I have to replace the inner bearing and the crush sleeve. Inexpensive and locally available, so no bigggie.

Good Luck!

 

HI All,
Apologies for resurrecting this thread, but it is relevant. I finally got my car back yesterday from No Coast Diff here in Tempe, went ahead and had the differential rebuilt:
Detroit TruTrac Posi
3.42 gearing (was about 2.5)
new Mosier Engineering axles
all new seals
I've just started the 500 mile break in, but I already have 2 questions:
1. The Speedometer is off now (reads about 90 when I'm only doing about 60). sorry, I'm confused, how does the diff rebuild affect this? Speedo cable is attached at the transmission, right? I assume I need to get a different speedo gear?
2. I can now only get up to about 50 or 60 mph with the engine rpm spiking at 2900 to 3k rpms! Ugh, so apparently I need another gear. Can you guys tell me what 4 speed transmission is most closely matched to my existing TH350 (reasonably seamless replacement)?

Many thanks, you guys are the best, greatly appreciate this forum!!
james

 

1. Yes, the lower rear gears mean the driveshaft and crankshaft are spinning faster than before when driving at a given speed. Replacing the speedo gear in the transmission (or installing a converter box between the trans and speedo cable) are needed to recalibrate the speedometer to the new rear gears.

2. The TH200-4R is a popular swap.

 

You just can’t help waiting for that shift into a higher gear, but it doesn’t happen!

200-4R is a good solution, do your research here and anywhere else.

FWIW - Blue car w/ 3.90 tachs 2300 @ 70mph, red car w/ 3.08 tachs 1800 rpm @ 70mph, you would end up roughly in the middle w/ 3.42.

​​​​​​……

 

Your car is a perfect candidate for a 2004R. As long as you have no plans for big power upgrades, a junkyard trans in good condition will live once you upgrade the valve body and servo.

The lower first gear of the 2004R will make those 3.42 gears feel like 4.10s, the overdrive and lockup converter will make the car cruise basically the way it did before the gear swap. Win win!

 

Thank you all for the excellent responses!
Bccan, You have it exactly right; I just keep waiting for that next gear to come, it never does...

I thought I would be okay with the higher RPM's on highway, but no, it's nerve wracking. I'll be looking into a 2004R asap!

james

 

I went with overdrive and 3.55 gears.... I love it...Tried 3.73 and over drive...But just too much for my liking .It really is a customer expectation deal. I like burn outs but do not need the acceleration feel after it is done. Smoke is nice then i putt alone after that...LOL I have race cars way faster so no street car is going to overly impress me anymore.
YOU will like the OD... make the car MUCH more derivable and you will drive it more and enjoy it more in my opinion
Jim
JD

 

As said, a completely direct swap, just move the crossmember back to the TH400 position. As said, probably a good shift kit like the big Transgo with a servo or CK shift kit and servo on a good stock trans is good enough. A new stator tube if it shows any wear. The usual stock non performance D9 converter stalls about 300 rpm higher than your current TH350, it will help off the line even more with the 2.74 first. There are various automatic controls for the lock up torque converter, it drops rpm another 300 rpm locked up. I used that exact combo in my 88 CSC, with a 26.3" tire, 1750 rpm at 60 mph and around 2050 rpm at 70 mph. You will be about 100 rpm lower with the stock height tire for your 76. This calculator is extremely accurate and is already programmed with the 2004R and the converter locked up.
http://tech.oldsgmail.com/ch_axle.php
Here is the speedometer calculator, you probably need the 10 tooth green gear. I have found most have either the black or brown small gears.
https://www.tciauto.com/speedometer-gear-calculator
Good luck, do this, add dual exhaust if you haven't. It will be a great, fun cruiser.