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79 DeVille Phaeton, 57 Eldo Biarritz, 79 Fleetwood d'Eleganc
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Discussion Starter #1 (Edited)
My 79 Deville is in the body shop patiently waiting for new paint.
While it is there I have been buying the needed parts for the restoration, hood, grille, new front bumper etc.
I was thinking about upgrading the rear end from the open it now has.
I don't know the ratio but guessing perhaps 2.73
It appears from my digging that the rear end in this car is the same as my 75 Buick LeSabre.
It believe it is a 10 cover 12 bolt ring gear 8.875 unit. I am not sure if is a 28 or 30 spline (the Buick is 30)
There is no after market support for these units and parts are really hard to find.
On my Buick I was able to find after a long search + big $$$, a rebuilt posi unit and used 2.93 gears,
along with a rear sway bar it was a real improvement.
So my question is does anyone know where I can buy a decent posi unit for the 79 Deville?
Can anyone confirm whether the splines are 28 or 30?
Did only the Fleetwood's, Limo's and commercial chassis come with a posi rear?
Any other help advice would be appreciated.
Cheers Robert
 

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77 CDV, 06 DTS III, 69 FWB
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10,022 Posts
Limited slip was an option for 1979 Cadillacs. You might look for the 3.08 rear end that was standard in CA and trailer package-equiped cars.
 

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1979 Sedan deVille d'Elegance
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For 1979 Cadillacs the limited slip rear axle and trailer towing package seem to be pretty rare options - my 1979 Sedan deVille has both. Next time car is in the air I can ask mechanic about the technicalities if you need. :)
 

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1979 Phaeton Coupe, 1990 Brougham d'Elegance 5.7 liter
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4,416 Posts
The standard rear end was 2.28. Limos got 3.08. I read that 2.73 was also available, but I don't know what it came on. Limited slip was a $70 option. The rear sway bar was included with the rear ride leveler option.

I'm curious, what was wrong with your old hood, grille, and bumper? They looked fine in your pictures.
 

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1979 Sedan deVille d'Elegance
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The standard rear end was 2.28. Limos got 3.08. I read that 2.73 was also available, but I don't know what it came on. Limited slip was a $70 option. The rear sway bar was included with the rear ride leveler option.

I'm curious, what was wrong with your old hood, grille, and bumper? They looked fine in your pictures.
If you have a 1979 Cadillac brochure look on the specifications page for a thorough rundown on rear end availability. :)
 

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79 DeVille Phaeton, 57 Eldo Biarritz, 79 Fleetwood d'Eleganc
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139 Posts
Discussion Starter #6
The standard rear end was 2.28. Limos got 3.08. I read that 2.73 was also available, but I don't know what it came on. Limited slip was a $70 option. The rear sway bar was included with the rear ride leveler option.

I'm curious, what was wrong with your old hood, grille, and bumper? They looked fine in your pictures.
The old hood had rust coming through at the outer right side passenger hinge (it is common most of the hoods I looked at had the same issue)
The grille chrome was starting to peal away inside the little "egg crates", and the front bumper had a nasty scub below the passenger headlight.
The parts from pic and pull when they become available around here are too cheap not to upgrade. I just missed and entire radio CB unit with correct antenna, in a 79 Eldorado that would have been great!

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Back to the rear end, does anyone know or can confirm if they are 28 or 30 spline axles?
Of the 3 Deville's that ended up in local wreckers here none had a posi.
Finding a 77-79 caddy with a posi rear end locally may prove to be both challenging and time consuming!
 

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79 DeVille Phaeton, 57 Eldo Biarritz, 79 Fleetwood d'Eleganc
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139 Posts
Discussion Starter #8
My digging on the inter web says our rear ends are specific to 77-79.
Because the car is crammed in the body shop now it is impossible to get a pic of the rear end to confirm with the rear end guru on the V8Buick site.
It looking like this is the same rear end used in my 75 Buick, where there is no aftermarket support and parts are a nightmare to find.
Rock Auto incorrectly shows they have lots of gear ratios for my 79 Phaeton 3.08 3.42 etc but they are for a 8.5 rear that is not in my car.
I would be very happy with a posi unit and 2.73, 2.93 or 3.08 gears
 

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1980 FBC
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3,163 Posts
A posi makes a world of difference on these cars as you already found out. I got a 2.28 posi out of a heavily optioned '78 which also had fuel injection.

I have not seen any other ratio in a 77-79 than 2.28. Anything higher kills highway economy.

I put an '84 Caprice 2.73 under my '77 some years back. It took a lot of finagling to get the Cadillac flange to play nice with the Chevy pinion.

Right now I'm living with the open factory 2.41 in my 1980. IIRC they are lighter than the older ones. I need to figure out if I'm going to go ahead with the 4 speed automatic conversion or just swap the rear and existing 2 speed Turbo-400. Yes, it is a 2 speed TH-400. Used to be 3 speeds.

If you stick a 3.42 or 3.41 I forget from an HT-4100 car under a 425 powered car it will fly, right to the gas station. Fun between stoplights though. No hope to get the tires to hook up from a dead stop though.

Good luck with finding what you need!
 

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1977 Coupe (blue), 1977 Coupe (yellow), 1977 Coupe (green)
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Because I could not answer you off the top of me cranium, I went to my shop and checked an axle I have from a 77 Brougham. They had 30 splines. According to the SM, it appears the Sevilles used the same carriers. This would allow you another gear set - 2.56:1. The Limos and the CC chassis use the same housings, however, " they contain larger pinion and wheel bearings, thicker housing walls and bigger axle shafts to provide for higher axle loading imposed by these heavier vehicles". Very Interesting. --- Limos and CC chassis have 3.08:1 gear sets. From this description I hazard a guess that perhaps the limos and CC vehicles used a carrier like you have in the '75 Buick which has 28 splines on the axle shafts and a much beefier axle diameter. They shortened the shafts to fit in the shorter housings.
Some years back I had correspondence with Morrison and they said that they could produce replacement axles for my Brougham. Instead of going that way, I changed out the complete rear.
 

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79 DeVille Phaeton, 57 Eldo Biarritz, 79 Fleetwood d'Eleganc
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Discussion Starter #11
So I should be looking for a 30 spline axled 8.875 rear end?
I just counted the splines on my extra Buick axle and it is 31, so it looks like the Buick posi units won't interchange.
 

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95 FWB 81SDV 96 FWB 94 Fleetwood
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Thank You Deville33... I have a 77-79 posi rear in my car . IT came with 2.28 gears. I put a set of 2.56 gears in and did not know what they came in ... Now I know it was a Seville.. They came with 28 and 30 spline... The 30 spline were in the limo's... If anyone needs a new 30 spline axle I have one.. Dave Menard
 

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79 DeVille Phaeton, 57 Eldo Biarritz, 79 Fleetwood d'Eleganc
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Discussion Starter #13
Doing a little more research and it appears that the rear end in my Phaeton is a 8.875 10 bolt cover 10 bolt ring gear 2.28 open and the axle spine count is 28,
So at this point I don't think a posi unit from a Limo, or Fleetwood will interchange without changing the housing is that correct?
It for certain will not interchange with the 8.875 rear end in my 75 LeSabre as that is a 10 bolt cover 12 bolt ring gear 2.73 with a 31 count axle spline.
 

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1979 Sedan deVille d'Elegance
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I was reviewing this thread and realized I posted two previous comments although I wish I knew what in the devil everyone is talking about with splines and bolts and ratios...would someone be kind enough to break this enigma code and explain what this all means in an understandable way? :)
 

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1980 FBC
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It is my understanding the 77-79 Deville/Fleetwood has 12 bolt guts in a 10 bolt housing. If you have a 2.28 gearset the highest you can go is 2.56 due to pinion thickness.

I could be mistaken..........

Those older Buke axles sound like a nice retrofit. Too bad they are like hen's teeth to find anymore.

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Thank You Deville33... I have a 77-79 posi rear in my car . IT came with 2.28 gears. I put a set of 2.56 gears in and did not know what they came in ... Now I know it was a Seville.. They came with 28 and 30 spline... The 30 spline were in the limo's... If anyone needs a new 30 spline axle I have one.. Dave Menard
Do you remember where you sourced a crush sleeve for the job?

I've just recently been burned by cheap knockoff auto parts and would like to avoid that again.

TIA.
 

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1977 Coupe (blue), 1977 Coupe (yellow), 1977 Coupe (green)
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The crush sleeves should be available from GM. If you buy a aftermarket gear set the sleeves may or may not come with the kit.
You should use a new nut on the pinion also. May not be necessary, but it is recommended. Getting the proper settings on the pinion pre-load can be touchy. Make sure you have all the proper tools to do the job. Do you have the pinion depth check tool?
If you take a carrier out of another housing, take everything which may be useful. We save all the shims and parts so they are available for the next job.
 

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I was reviewing this thread and realized I posted two previous comments although I wish I knew what in the devil everyone is talking about with splines and bolts and ratios...would someone be kind enough to break this enigma code and explain what this all means in an understandable way? :)
When you drive your car down the road in 3rd gear your engine is turning faster than your final drive out of the rear axles. Most cars are set up so that highway speed (55 miles per hour) has the engine turning around 2000 revolutions per minute (rpm). The exception to this could be a vehicle equipped with overdrive. The manufacturer offers a gear set which should accomplish this as standard. For the vehicles we are discussing the gear set was the 2.28:1. California cars got the 2.73:1 gear set.
Using the 2.28:1 as an example, for every single rotation of your tire, your engine turns approximately 2 and 1/4 turns. For the California cars it was 2 and 3/4 turns. If someone wanted to use their Cadillac to tow a boat or heavy loads, as in a limo, they were offered the 3.08:1 gear set.
Splines: In order to turn the wheels there has to be a means to conduct the drive from the differential to the axles. The diameter of the axle shaft determines how many splines you can use. The axle shaft has serrations which match with serrations in the side gears of the differential. These serrations are called splines.
The drive shaft drives the pinion gear, which mates to the ring gear, which is bolted to the carrier or differential case. This is what we speak of when we say it is a 10 bolt or 12 bolt carrier. Smaller gear sets have 10 bolts because the ring diameter will only allow ten (10) grade 8 bolts to hold the ring in place.
In the center of all this is a Pinion shaft. The pinion shaft fits snugly in the center of the case. It locates two (2) pinion gears, which locate on either end of the shaft. The shaft is held in place by a small lock screw. These pinion gears are sometimes referred to as spider gears. They allow the transfer of power to one wheel or the other. In order to go around a corner one wheel has to turn faster than the other. These gears allow that to happen.
These pinion gears mate with the two (2) side gears, which align with the axle shaft tubes of the housing.
These side gears mate with the splines in the axle shafts.
The axles are held in place by what are called "C" locks or clips.
All these parts are supported by opposing conical bearings. All have to be properly assembled in order for the unite to perform as it should.
I hope this helps you understand the topic better, but a illustration would help much more. If you have a Service manual, they have illustrations and descriptions which are perhaps better at describing this.
 

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1980 FBC
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Thanks, I forgot about the nut. Last time I made a jam nut out of another nut because the first nut didn't seem tight enough to stay locked. I don't remember why it landed up like that but I do remember some pinion flanges are deeper than others by a small amount.

I can borrow a set of dial indicators. Been there before but it has been a long time.

This would be for replacing a 2.28 gearset with 2.56 in a posi.
 

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2014 ELR
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It is my understanding the 77-79 Deville/Fleetwood has 12 bolt guts in a 10 bolt housing.
It's also my understanding that the 80-81 is the same. I don't know where guyR heard differently, however.
 

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1979 Sedan deVille d'Elegance
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When you drive your car down the road in 3rd gear your engine is turning faster than your final drive out of the rear axles. Most cars are set up so that highway speed (55 miles per hour) has the engine turning around 2000 revolutions per minute (rpm). The exception to this could be a vehicle equipped with overdrive. The manufacturer offers a gear set which should accomplish this as standard. For the vehicles we are discussing the gear set was the 2.28:1. California cars got the 2.73:1 gear set.
Using the 2.28:1 as an example, for every single rotation of your tire, your engine turns approximately 2 and 1/4 turns. For the California cars it was 2 and 3/4 turns. If someone wanted to use their Cadillac to tow a boat or heavy loads, as in a limo, they were offered the 3.08:1 gear set.
Splines: In order to turn the wheels there has to be a means to conduct the drive from the differential to the axles. The diameter of the axle shaft determines how many splines you can use. The axle shaft has serrations which match with serrations in the side gears of the differential. These serrations are called splines.
The drive shaft drives the pinion gear, which mates to the ring gear, which is bolted to the carrier or differential case. This is what we speak of when we say it is a 10 bolt or 12 bolt carrier. Smaller gear sets have 10 bolts because the ring diameter will only allow ten (10) grade 8 bolts to hold the ring in place.
In the center of all this is a Pinion shaft. The pinion shaft fits snugly in the center of the case. It locates two (2) pinion gears, which locate on either end of the shaft. The shaft is held in place by a small lock screw. These pinion gears are sometimes referred to as spider gears. They allow the transfer of power to one wheel or the other. In order to go around a corner one wheel has to turn faster than the other. These gears allow that to happen.
These pinion gears mate with the two (2) side gears, which align with the axle shaft tubes of the housing.
These side gears mate with the splines in the axle shafts.
The axles are held in place by what are called "C" locks or clips.
All these parts are supported by opposing conical bearings. All have to be properly assembled in order for the unite to perform as it should.
I hope this helps you understand the topic better, but a illustration would help much more. If you have a Service manual, they have illustrations and descriptions which are perhaps better at describing this.
Wow! What a great opportunity to learn! Appreciate very much your time to explain this! :)
 
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