Rear end research
February 13th, 2014, 08:06 PM
#1
Rear end research
I am looking into the rear end options that are available for my cutlass. I found out that some A Body parts are interchangeable and I thought rear ends may fall into that category so I've started snooping around. Here is what I found so far that seems to be good information but I'm not sure.
http://www.oldsmobility.com/old/interchange.htm
When dealing with used rear ends, how does one know what the gear ratio is before buying?
What is the typical move regarding replacing rear ends? When you all have changed out the rear end, what is it that you've done?
http://www.oldsmobility.com/old/interchange.htm
When dealing with used rear ends, how does one know what the gear ratio is before buying?
What is the typical move regarding replacing rear ends? When you all have changed out the rear end, what is it that you've done?
February 21st, 2014, 01:24 PM
#2
Okay, apparently no one has done this... :-). It's probably because I just posted the link. My apologies. Seriously though, I want to make sure I don't spend unnecessary money on a new rear housing if I don't have to...
Now that the suspension has been inspected and found in good shape, I am considering going from the rear to the front (rear end, then transmission, then engine work) - it seems that I would get the most fun out of it right away that way (from what I've gathered here over the months)...
What I've been told from a rear view of my differential is that I have an 8.2 (quite possibly because of a bad picture or whatever I am certainly ready to assume the reason for it), but then I came across this post: https://classicoldsmobile.com/forums...swap-71-s.html
In that post it says that if your differential has one "rib" or converging "ribs" it is an 8.2 but if it looks like the picture posted it is an 8.5 (2 parallel ribs). So here is the picture that was posted (with the little diagrams too):
8.2:
8.5:
And here is my differential: Right (passenger) side view
Rear view
Left (driver) side view
Can someone please help me identify what I have so that I know whether or not I have to buy a different rear housing for lower gearing?
A second question (and maybe the whole reason people swap instead of replacing gears) is, would it be cheaper to swap the gears if the housing allows for a 3.42/3.55/3.73 setup rather than to swap the whole thing or is it more involved and thus the reason many people swap the whole thing?
Now that the suspension has been inspected and found in good shape, I am considering going from the rear to the front (rear end, then transmission, then engine work) - it seems that I would get the most fun out of it right away that way (from what I've gathered here over the months)...
What I've been told from a rear view of my differential is that I have an 8.2 (quite possibly because of a bad picture or whatever I am certainly ready to assume the reason for it), but then I came across this post: https://classicoldsmobile.com/forums...swap-71-s.html
In that post it says that if your differential has one "rib" or converging "ribs" it is an 8.2 but if it looks like the picture posted it is an 8.5 (2 parallel ribs). So here is the picture that was posted (with the little diagrams too):
8.2:
8.5:
And here is my differential: Right (passenger) side view
Rear view
Left (driver) side view
Can someone please help me identify what I have so that I know whether or not I have to buy a different rear housing for lower gearing?
A second question (and maybe the whole reason people swap instead of replacing gears) is, would it be cheaper to swap the gears if the housing allows for a 3.42/3.55/3.73 setup rather than to swap the whole thing or is it more involved and thus the reason many people swap the whole thing?
February 21st, 2014, 02:58 PM
#3
Registered User
Join Date: Oct 2009
Location: Erie,PA
Posts: 3,814
You have a 12-bolt O-axle,which is correct for a 68. The big problem is that there is currently NO aftermarket gears available for that rear.
The best option is to find an 8.5" 10-bolt,from a 71-72 Cutlass/442,or 71-72 Skylark/GS,and build that. You will have to shorten your driveshaft,when switching from the O-axle to the 8.5".Other than that,it is a direct bolt-in swap. There are plenty of aftermarket parts available for those,and they are plenty strong.
The best option is to find an 8.5" 10-bolt,from a 71-72 Cutlass/442,or 71-72 Skylark/GS,and build that. You will have to shorten your driveshaft,when switching from the O-axle to the 8.5".Other than that,it is a direct bolt-in swap. There are plenty of aftermarket parts available for those,and they are plenty strong.
February 21st, 2014, 03:13 PM
#4
Just an Olds Guy
Join Date: Jul 2008
Location: Edmonton, AB. And "I am Can 'eh' jun - eh"
Posts: 24,525
What's wrong with the gears in the diff you have?
February 21st, 2014, 08:55 PM
#5
507Olds: what is an o-axle? Why don't they make any better gearing for it? Do you know any of the best places to find those rear ends you mentioned? How much should I expect to pay for a 71-72 Cutlass/442,or 71-72 Skylark/GS rear end? Do you know of anyone who has one for sale? Is it the case that I would have to buy all the guts for it after that?
Allan R: I have no idea what gears are in my rear end. It is an absolute dog off the line (and it only spins one wheel when i can get the tires to break loose around a corner), so I figured it was the 2.56 gears (which seemed to be common for that year with my configuration [low compression, 2bbl, etc]). If I pull off the rear differential cover, what should I look for to see what the gearing is? What is involved in replacing the cover (gaskets, differential oil, etc)?
Allan R: I have no idea what gears are in my rear end. It is an absolute dog off the line (and it only spins one wheel when i can get the tires to break loose around a corner), so I figured it was the 2.56 gears (which seemed to be common for that year with my configuration [low compression, 2bbl, etc]). If I pull off the rear differential cover, what should I look for to see what the gearing is? What is involved in replacing the cover (gaskets, differential oil, etc)?
February 21st, 2014, 11:02 PM
#6
Registered User
Join Date: Feb 2013
Location: Phoenix, AZ
Posts: 14,042
The O-Type 12 bolt is a unique rear that was built by Oldsmobile from 1967 to 1970. It has a 12 bolt cover with a 10 bolt ring gear. It is often confused with the Chevrolet 12 bolt rear that has a 12 bolt cover and a 12 bolt ring gear, but they are entirely different things. It has an 8.5" ring gear that is different than the gears in the later 71-up 8.5" corporate 10 bolt rear. As you see, it is a fairly unique rear and as such there aren't many aftermarket parts for it.
Several companies used to make gear sets for it in several ratios (3.42, 3.90, 4.10 and maybe another lower ratio) but there is such a small market and little demand for them that it apparently wasn't profitable for them to manufacture them.
A rear off of any 68-72 A-Body will bolt up to your car, but these could be the desirable and pricey Chevy 12 bolt, or the less desirable 8.2" 10 bolt, depending upon the vehicle:
Chevrolet Chevelle, Malibu, El Camino, Monte Carlo
Buick Skylark, GS
Pontiac Le Mans, GTO, Tempest, Grand Prix
If you remove the cover on the rear, there are numbers and letters stamped into the edge of the ring gear. There will be a GM part number and then something like 41 A 16 which is the number of teeth on the ring gear and the pinion gear. Do some simple math to determine the ratio: 41 divided by 16 = 2.56
You can get a replacement gasket from most parts stores or just use some good silicone sealer around the perimeter of the cover. Refill with regular gear oil from the local parts store.
I don't think it's really worthwhile to pull the cover to check the ratio as you already figured out its likely a 2.56 rear. That was the standard no-cost rear for that era (other ratios were optional for an additional cost).
Whew, that's a lot of info to type. I hope it helps you.
Several companies used to make gear sets for it in several ratios (3.42, 3.90, 4.10 and maybe another lower ratio) but there is such a small market and little demand for them that it apparently wasn't profitable for them to manufacture them.
A rear off of any 68-72 A-Body will bolt up to your car, but these could be the desirable and pricey Chevy 12 bolt, or the less desirable 8.2" 10 bolt, depending upon the vehicle:
Chevrolet Chevelle, Malibu, El Camino, Monte Carlo
Buick Skylark, GS
Pontiac Le Mans, GTO, Tempest, Grand Prix
If you remove the cover on the rear, there are numbers and letters stamped into the edge of the ring gear. There will be a GM part number and then something like 41 A 16 which is the number of teeth on the ring gear and the pinion gear. Do some simple math to determine the ratio: 41 divided by 16 = 2.56
You can get a replacement gasket from most parts stores or just use some good silicone sealer around the perimeter of the cover. Refill with regular gear oil from the local parts store.
I don't think it's really worthwhile to pull the cover to check the ratio as you already figured out its likely a 2.56 rear. That was the standard no-cost rear for that era (other ratios were optional for an additional cost).
Whew, that's a lot of info to type. I hope it helps you.
Last edited by Fun71; February 21st, 2014 at 11:15 PM.
February 22nd, 2014, 02:09 AM
#7
Registered User
Join Date: Jan 2014
Location: Camdenton, MO
Posts: 342
Since diff ratios typically come in standard numbers like 2.56, 2.73, 3.08 etc. you can usually mark the driveshaft and wheels, jack up the rear, count the number of driveshaft revolutions it takes to get the wheels to make 1 revolution. Make sure both wheels turn the same.
As was pointed out, any 68-72 A body rear will fit. I'm running an 8.2 chevelle rear in mine. Going to a different rear can affect the driveshaft length.
As was pointed out, any 68-72 A body rear will fit. I'm running an 8.2 chevelle rear in mine. Going to a different rear can affect the driveshaft length.
February 22nd, 2014, 11:43 AM
#8
Just an Olds Guy
Join Date: Jul 2008
Location: Edmonton, AB. And "I am Can 'eh' jun - eh"
Posts: 24,525
If you remove the cover on the rear, there are numbers and letters stamped into the edge of the ring gear. There will be a GM part number and then something like 41 A 16 which is the number of teeth on the ring gear and the pinion gear. Do some simple math to determine the ratio: 41 divided by 16 = 2.56
I don't think it's really worthwhile to pull the cover to check the ratio as you already figured out its likely a 2.56 rear. That was the standard no-cost rear for that era (other ratios were optional for an additional cost).
I don't think it's really worthwhile to pull the cover to check the ratio as you already figured out its likely a 2.56 rear. That was the standard no-cost rear for that era (other ratios were optional for an additional cost).
His description of the performance shows it's clearly an open differential though.
February 22nd, 2014, 11:56 AM
#9
Registered User
Join Date: Dec 2011
Posts: 7,286
if it's not a posi, then jack ONE wheel and count driveshaft turns to attain TWO turns of the one wheel, which is one turn of the ring gear, which is the same as one turn of two wheels.
Go see the youtube videos about how differentials work if this is not clear.
This is one of my favorites, not sure it goes into posi/ limited slip/ sure-track/ whatever
many years later the same concept applies.
If BUYING a differential, removing the cover and inspecting the innards is mandatory, how else can you know what you are buying? Might as well read the tooth counts stamped into the ring gear. Pinions can really only be about 9 thru 14 teeth, and so that will be the range of the smaller number. At least one of the numbers is almost always a prime number- 39, 41, etc. so that the wear is spread out evenly over all the teeth over the life of the part, maximizing the longevity.
So if you see "39 10" stamped in the ring gear and it takes just short of 4 turns to get one spin of the ring gear, you have a 3.90:1 ratio.
Go see the youtube videos about how differentials work if this is not clear.
This is one of my favorites, not sure it goes into posi/ limited slip/ sure-track/ whatever
many years later the same concept applies.
If BUYING a differential, removing the cover and inspecting the innards is mandatory, how else can you know what you are buying? Might as well read the tooth counts stamped into the ring gear. Pinions can really only be about 9 thru 14 teeth, and so that will be the range of the smaller number. At least one of the numbers is almost always a prime number- 39, 41, etc. so that the wear is spread out evenly over all the teeth over the life of the part, maximizing the longevity.
So if you see "39 10" stamped in the ring gear and it takes just short of 4 turns to get one spin of the ring gear, you have a 3.90:1 ratio.
February 23rd, 2014, 07:43 PM
#10
February 23rd, 2014, 08:11 PM
#11
Registered User
Join Date: Oct 2009
Location: Erie,PA
Posts: 3,814
That is a good example if what NOT to get.
That is an 8.2" Chevy rear,with a 2-series carrier,and a ring gear spacer. That rear has C-clips,not bolt-in axle shafts,like what you have now. Strength wise,it is not as good as the 8.5" 10-bolt,or the O-axle you have now.ring gear spacers do work fine in some rears,but not the 12-bolt Chevy,or the 8.2" Chevy. One reason being the spacer takes up the width of the ring gear journal in the carrier,leaving nothing for the ring gear to press onto,to keep it centered & prevent deflection under load.It also has smaller bolts,which have a greater chance of flexing & breaking.
It's not worth the $750.00,in my opinion.
That is an 8.2" Chevy rear,with a 2-series carrier,and a ring gear spacer. That rear has C-clips,not bolt-in axle shafts,like what you have now. Strength wise,it is not as good as the 8.5" 10-bolt,or the O-axle you have now.ring gear spacers do work fine in some rears,but not the 12-bolt Chevy,or the 8.2" Chevy. One reason being the spacer takes up the width of the ring gear journal in the carrier,leaving nothing for the ring gear to press onto,to keep it centered & prevent deflection under load.It also has smaller bolts,which have a greater chance of flexing & breaking.
It's not worth the $750.00,in my opinion.
February 23rd, 2014, 09:13 PM
#12
Registered User
Join Date: Feb 2013
Location: Phoenix, AZ
Posts: 14,042
February 24th, 2014, 02:10 PM
#14
That is a good example if what NOT to get.
That is an 8.2" Chevy rear,with a 2-series carrier,and a ring gear spacer. That rear has C-clips,not bolt-in axle shafts,like what you have now. Strength wise,it is not as good as the 8.5" 10-bolt,or the O-axle you have now.ring gear spacers do work fine in some rears,but not the 12-bolt Chevy,or the 8.2" Chevy. One reason being the spacer takes up the width of the ring gear journal in the carrier,leaving nothing for the ring gear to press onto,to keep it centered & prevent deflection under load.It also has smaller bolts,which have a greater chance of flexing & breaking.
It's not worth the $750.00,in my opinion.
That is an 8.2" Chevy rear,with a 2-series carrier,and a ring gear spacer. That rear has C-clips,not bolt-in axle shafts,like what you have now. Strength wise,it is not as good as the 8.5" 10-bolt,or the O-axle you have now.ring gear spacers do work fine in some rears,but not the 12-bolt Chevy,or the 8.2" Chevy. One reason being the spacer takes up the width of the ring gear journal in the carrier,leaving nothing for the ring gear to press onto,to keep it centered & prevent deflection under load.It also has smaller bolts,which have a greater chance of flexing & breaking.
It's not worth the $750.00,in my opinion.
. I have heard the term c-clip and I have a general idea of what that is (something that holds the axles in) but beyond that everything else is scrambled eggs to me (I can't say Greek, because I have a pretty sound understanding and usage of Greek)...anyway, would you mind helping me identify those parts you mention, when (or if) you have time?Do you have a (hopefully illustrated) picture of an example of what would be a good purchase decision?
Sorry, Octania, I get the reference, butt...that's not the type I'm referring to...
February 25th, 2014, 05:46 AM
#15
Registered User
Join Date: Jan 2014
Location: Camdenton, MO
Posts: 342
I wouldn't rule out the 68-72 chevy 8.2 with c clips unless you have big power and sticky tires. Around here they're a lot easier and cheaper to find that the 12 bolt or BOP rears.
But good catch on the spacer by 507OLDSS
But good catch on the spacer by 507OLDSS
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