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Based upon my research in this CO forum & elsewhere, in particular statements by Dave (monzaz), I believe the differential in my 1971 Cutlass Supreme 350cid convertible is a 28 spline 8.5, 2.73 gear ratio. I don't have any reason to open the differential housing - there are absolutely no leaks, no slipping, no whining & it's solid. I've checked the fluid on two different occasions and it's surprisingly clear (probably was changed just prior to my purchase July 2018). It's my understanding this stock differential is quite beefy and well built. Looking for anyone/someone to confirm based upon the following:
10 Bolt - 2-straps, 4 bolts
Casting - 407296
Casting - CE (I believe represents open limited-slip differential)
Casting - CFD
I've looked on the bottom (not the top) of the passenger side axle tube for a stamped number (i.e. SA 0237) but I haven't found one, yet. Perhaps it's on the top and I need to look better. Possibility the RH axle tube is not the original axle tube, as well - not sure. The vehicle has matching VIN derivatives (TH350 transmission & block).
Thanks!
Last edited by Vintage Chief; January 2nd, 2019 at 04:53 PM.
Reason: update hyperlink
Some of those casting numbers are just foundry ID and aren't specific to any particular rear (CFD = Central Foundry Division).
For ID purposes, the 8.5" 10 bolt rear has two parallel ribs on the sides of the center section and a threaded boss on top where the brake hose mounts.
My original 71 rear has a block "O" on each side of the forward part of the center section where the axle tubes attach.
And here's a pic of the axle tube stamping (it is on front of the right side tube):
Last edited by Fun71; January 2nd, 2019 at 04:50 PM.
And here's a pic of the axle tube stamping (it is on front of the right side tube):
Thanks. I'll take a better look for stamp on the axle tube. On another thread, a poster stated it was located on the back of the passenger axle tube; although, I did look on the front, as well.
From the rear cover picture and the casting surrounding it all the tell tail signs for you having the original 8.5 10 bolt is correct.
I think by the time 1971 corp rears rolled out most axle tubes were all being stamped where the CHEVYs were normally stamped. passenger tube front facing the front of the car. (AS Pictured by FUN71)
10 bolt 8.2 Pontiac were generally on the back side of the driver tube
10 bolt and 12 bolt Chevy were generally front of the passenger side axle tube facing front
10 bolt 8.2 and 8.25 Buick 1965-1970 driver side tube facing the GROUND
12 bolt cover 10 bolt ring olds 8.5 were passenger side facing the back of the car UPSIDE DOWN... That is the way we normally found them
If the rear is Limited Slip... you can always do the old spin test. mark the tire mark the yoke and casting for reference spot and count how many turns the yoke spins with ONE revolution of the tire. 2.73.... etc.
From the rear cover picture and the casting surrounding it all the tell tail signs for you having the original 8.5 10 bolt is correct.
I think by the time 1971 corp rears rolled out most axle tubes were all being stamped where the CHEVYs were normally stamped. passenger tube front facing the front of the car. (AS Pictured by FUN71)
10 bolt 8.2 Pontiac were generally on the back side of the driver tube
10 bolt and 12 bolt Chevy were generally front of the passenger side axle tube facing front
10 bolt 8.2 and 8.25 Buick 1965-1970 driver side tube facing the GROUND
12 bolt cover 10 bolt ring olds 8.5 were passenger side facing the back of the car UPSIDE DOWN... That is the way we normally found them
If the rear is Limited Slip... you can always do the old spin test. mark the tire mark the yoke and casting for reference spot and count how many turns the yoke spins with ONE revolution of the tire. 2.73.... etc.
Just one more thing on that picture with the drive shaft. If you have any kind of Power or going to upgrade power in that car... WATCH out for the drive shaft you have... It is a 2 piece tube unit. They are held together with impregnated rubber and over this amount of time can be dried out and deform and or let go causing the drive shaft to become out of phase at the front and rear u -joint.
That smaller diameter tube goes inside the larger diameter tube and hardened rubber like in your control arm bushings is what holds them to each other.... It was to control harmonics from the rear end through the drive line. Most guys just get a solid steel one piece these days.
Something to write down in case you ever start to get vibration issues.
Well, it isn't a limited slip, it's an open carrier differential. You'd think I would have noticed having been under the car numerous times, although I've been focusing on addressing engine updates and not the drive train at this point.
LH Driver's Side Wheel Locked in a stationary position (Open Differential) yields:
Rear Differential Gear Ratio = 2:56 (41:16 Gear Teeth) - Open Differential
Passenger Side Axle Tube Stamp: R2 0180
Information in my 1971 Oldsmobile CSM (p. 0-6, Fig. 10A - Differential Identification Code), demonstrates the STD Differential should be an R2 (R3 would be an Anti-Spin Differential).
It is unclear what 0180 references. I'm going to have another look to see if I can find an "O" on each side of the forward part of the center section where the axle tubes attach as Kenneth pointed out. But, at this point, I think I'm assured I have a 2:56 open carrier differential which came STD on this vehicle during manufacturing.
Thanks to both of you for the assistance.
Last edited by Vintage Chief; January 4th, 2019 at 05:54 AM.
Yep, R2 is 2.56:1 ratio, open differential.
I think the other numbers are the manufacturing division and the date code.
Right on. Thanks for your assistance. I'm pretty gravy with this, also. Since this is a convertible (it has only ever been titled in the state of GA prior to my purchase), I'm betting it's never seen a 'hard' day in it's entire life (matching VIN derivatives) and it'll certainly get better gas mileage.
Just one more thing on that picture with the drive shaft. If you have any kind of Power or going to upgrade power in that car... WATCH out for the drive shaft you have... It is a 2 piece tube unit. They are held together with impregnated rubber and over this amount of time can be dried out and deform and or let go causing the drive shaft to become out of phase at the front and rear u -joint.
That smaller diameter tube goes inside the larger diameter tube and hardened rubber like in your control arm bushings is what holds them to each other.... It was to control harmonics from the rear end through the drive line. Most guys just get a solid steel one piece these days.
Something to write down in case you ever start to get vibration issues.
Jim
Hey Jim. I hadn't paid any attention. Thanks for the information. I won't be upgrading the driveshaft for any Power reasons. I'll be very content cruising in a convertible w/ a solid built 2.56 rear-end (at least it will get pretty fair gas mileage). Lots of different driveshafts out there on many different vehicles. I began developing a modest vibration in my 2003 F250 4x4 diesel. Checked the U-Joints, etc. and couldn't find a thing wrong. Finally, managed to discuss this with a good Ford diesel mechanic. He said about 125K-150K the slip yokes in the rear driveshaft sometimes develop vibration. I opened up the slip yoke, greased it up, reassembled and no more vibration.
FYI, back in the 80s I swapped the original 2.56 rear for a Chevy 12 bolt 3.55 rear. The in-town mileage stayed the same, but oh boy was the car a lot quicker and much more fun to drive.
FYI, back in the 80s I swapped the original 2.56 rear for a Chevy 12 bolt 3.55 rear. The in-town mileage stayed the same, but oh boy was the car a lot quicker and much more fun to drive.
Hence the moniker - Fun71 - Kenneth?
My father picked out my very first automobile - saying it was time (1969 - I was 16 years old) for me to own & drive my own car. My father was not a muscle car enthusiast, but oh did he love cars - specifically Buicks. Our family owned over the years (Buick Electra, Buick Wildcat, Buick LeSabre, Buick Invicta Station Wagon, and several more). When he showed me the car he had picked out and I was going to buy, I pee'd my pants. I think he saw himself wanting to own this car when he was 16 years old! My father's first car was a Lincoln V12 he purchased after WWII (Dad was in the Navy - made seven landing runs on Omaha Beach Normandy Invasion and survived to raise a family). My first car was a used gold 1967 Oldsmobile 442, nothing fancy - automatic on the column & bench seats. But, was I the envy of several of my friends. I paid (I should say I had a 3 year loan) of $1695.00 for the car and it didn't have one single blemish on the entire car - it was spotless. While in the USAF, I purchased a used 1972 Oldsmobile 442 (Blue+dual white stripes) in 1975. That vehicle absolutely screamed. After many other cars and years later, I felt compelled to own another Cutlass. These things are a dream to work on compared to any other modern(?) car.
Last edited by Vintage Chief; January 4th, 2019 at 12:43 PM.
January 2nd, 2019, 01:09 PM
