Brad Chapin

Looking to redo the rear end in my 72 cutlass supreme convertible.
From advice I have been given I値l likely switch to 3.42 gears and a posi.... outside of that I have no clue what else I値l need!
I know the stock axels are bolt in 28 spline.
What are the measurements I値l need when trying to find replacements?
Is there significant benefits to switching to 30 spline?

I知 new to everything so any parts lists, recommendations, opinions or general thoughts are welcome!

PS- I know that the specifics of proper installation of the gears, axels, etc are beyond my novice mechanical skills so I値l most likely have the work done by someone else- I壇 just like to piece together all of the parts I値l need over time so I can just drop the car pull of parts boxes off and let the professional do their work!

oddball

Our bolt in axles are fairly rare, so they're special order from all the axle manufacturers. The good thing is unless you're on slicks and >500 hp then there's no need to upgrade.
Brian Trick, 507olds on here and he's on Facebook, builds rear ends for folks and always does an excellent job. Monaz also builds rear ends. Sometimes it's more effective to just get a new rear built and swap it in.

70sgeek

FWIW, Assuming you're running a factory 3-speed automatic, if your car will see mostly street cruising/highway use, 3.42 gears may prove a bit steep at higher speeds unless you upgrade to an overdrive transmission. I had my '72 rebuilt with new Yukon 3.08 gears and posi, retaining the stock axles with all new bearings. No regrets - motor runs approx. 3000 rpm at 70 mph, very livable compared to a 3.23 I used to have which ran well over that by 60 mph.

Fun71

I agree with oddball that you don't need to replace the axle shafts unless you are planning to make a lot of power and run sticky tires.

Brad Chapin

Awesome! I appreciate the insight!
So in theory what all would have to be on a parts list for this project since I can use the factory axels?

Ring and pinion gears (3.08 sounds promising since it値l still have the stock 3 speed tranny for cruising)

Limited slip differential carrier (for 28 spline, GM 8.5 in if I致e been given correct information on the stock rear end)

Ok- that痴 all I know- what gets added to the parts list from there? Ha- thanks everyone!!

11971four4two

HEY Brad

I have had good luck with JIM


https://classicoldsmobile.com/forums...rs/monzaz.html

If you want to buy local I have a dealer account at proven force maple grove you could use.....

Fun71

Installation kit with bearings, seals, crush sleeve, pinion shims, carrier shims, marking compound, etc. Oh yeah, and gear oil and posi additive (if you're using a clutch style posi).

70sgeek

You will also need to use the right transmission speedometer gears to be sure it stays accurate for your gearing and tire size changes. One of them simply plugs inside a housing on the driver side of the transmission where the speedo cable plugs in, the other though, is a ring gear which is actually installed inside the transmission body.

this is where the factory service manual is your friend - it has a chart showing all the gear combinations for a given setup.

for instance, my gears are the #18 and #42 corresponding to my 3.08 gears and 245-60-R15 tires. Speedo is right on with my combination. The numbers correspond to the number of teeth the gears have.

I was lucky in that my drivetrain was already out of the car for rebuilding at the time the gears were installed so no major work involved to change them at that time.

cdrod

Two Thumbs up for Monzaz! I bought all my parts from him (3.42 gears, Eaton TrueTrac carrier, bearings & seals) and had a local shop do the install and set-up. I also had the local shop check the housing in their alignment jig to check for bent or crooked axle tubes. Monzaz was very helpful.