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I'm trying to figure out what this rear end is, and whether it's worth pursuing or just leave it. I've checked front and back of both axle housings and haven't been able to find any ID stamps yet.
This is in a 1970 Cutlass S, 350 with TH350, with rear sway bar and factory boxed lower control arms. It is limited slip with the gear ratio at 2.78. The cover was replaced by a previous owner, I haven't had it off (yet).
Is it an 8.5", and are other gear sets readily available for these? The 2.78 just won't cut it.
The front clip on the car is from a '72, so if this isn't a typical rear end for a '70 it may have come from the donor car.
That is only a 10 bolt cover and looks like a Corporate 8.5", not the Type O with a 12 bolt cover. Pull the cover and count the number of teeth on the ring and pinion, they were 2.73 not 2.78, if it is that ratio. Also look for C clips. Hopefully that is the Corporate 8.5" with bolt in axles from the 72. The carrier breaks are 2.56 and down, a spacer or new posi would be needed when swapping gears. The 2.73 and up carrier will take any decent ratio and you can reuse the posi, if it actually has one. There are endless, much more affordable gear ratios, many aftermarket posi and Moser axles as well for the Corporate 8.5".
Last edited by olds 307 and 403; January 4th, 2022 at 04:37 AM.
early 8.5 (71-72) that has a BOP 8.2 looking cover is deceptive. Still bolt in axle.. 2.73 pull cover as above and pull a tape measure dog ! That is 3 series once 2.73 Corp 8.5 verified... you can use that posi carrier on any gears like 3.08 higher numerically
Last edited by GEARMAN69; January 4th, 2022 at 07:26 AM.
That's what I was hoping, thanks! I'll have to wait a few days before I can get at it again to pull the cover. Have to play Tetris with the other cars again first!
It could very well be 2.73...five degrees isn't much when you're eyeballing the rotations. Truth will be in the visual.
Key identification for the 8.5" 10 bolt are the two parallel ribs on the sides of the center section, and the boss on top for mounting the brake hose. We definitely see the parallel ribs, but need confirmation of the mounting boss for a definitive ID.
You bought 1973-1977 - They are the most common A-body rears... They will not fit the 1972 and older.
Shoot some pictures of the rear we can ID it.
Will need the whole rear end shots with lower and upper control arm mounting areas in the pictures.
pictured below is a 1973-1977 rear
I think those photos are the '73-77. Would this be the boss you're referring to? Hard to get a decent photo with the diff in the car and full load on, using a brand new phone I don't want to scratch!!!
Last edited by VI Cutty; January 4th, 2022 at 05:33 PM.
Curiosity got the better of me and I was able to shuffle some stuff around to get at it. Ring is stamped 4668 41 15 GMC making it a 2.73 ratio. Hard to get an accurate measurement but looked like 8.5"
Yes those are bolt in.. see the holes in axle flange to stick your 3/8 socket and extension thru. See the nuts on the T bolt inside the brake shoe area holding the 4 bolt plate. If it was say a 71-81 2nd gen F body C clip one it would have not inside plate flange and just hex bolts pushed thru with nuts on backside of backing plate to hold it on (also no holes in axle flanges) vs this one the T bolts push thru from the other side and nuts go inside where you take off to get axles out.
As said you can see the flange and studs/nuts that hold the axle on, a big bonus. Excellent, it looks like they swapped in arguably the best factory axle from the 72. Brian Trick likes these as much as the factory 12 bolt in these cars and he builds rear ends for a living. It makes building the rear end about half the cost with way more options👍.
Last edited by olds 307 and 403; January 5th, 2022 at 06:01 AM.
As said you can see the flange and studs/nuts that hold the axle on, a big bonus. Excellent, it looks like they swapped in arguably the best factory axle from the 72. Brian Trick likes these as much as the factory 12 bolt in these cars and he builds rear ends for a living. It makes building the rear end about half the cost with way more options👍.
Gotta like it when things work out like that! Now to decide what gears to get for it and swap it into the good car. Thanks for the help, all!
I have been using later model 8.625" 10 bolt newer version good used Chevy truck gears from early 2000's models in some F body 8.5 rears lately and doing 30 spline axle upgrades with new Yukon style (OEM Eaton style Posi)
I dug out the other rear end I had mentioned...and of course, nothing is straightforward. Although I pulled this rearend from a 70 Cutlass S that I parted out a few years ago, I don't know if it was original to the car as the engine and transmission seem to have been sourced from a '69 full size. Anyhow...
This appears to be a limited slip gearing as both wheels turn in the same direction. The axle stamp is SA which seems to be an open diff as installed by the factory. All the SA differentials I see in the table over at Olds FAQ are 2.78 but this one spins out to 3.08 (verified by gear stamps). The gear stamping is 13A40 GM LS 28 7, and is 8.5" 10 bolt.
Am I correct in interpreting all this as being a Type O with 12 bolt cover, containing an upgraded 3.08 posi unit?
The 3.08 is still not the gearing I want with that 5 speed...which of the two rear ends here would be a better choice for putting the money and effort into?
Tough call, since you need a posi and gears for the 10 bolt, it will be slightly stronger. Cost is a tough one, depends if the clutches are toast, probably a wash between the two. That 3.08 12 bolt Type O posi will work with the aftermarket 3.42 and 3.90 gears, a huge savings over the aftermarket ones. The factory posi does not fit 71/72 Moser bolt axles, the aftermarket posi does. The 70 has stronger oiled bearing bolt in axles vs a 68 or 69 sealed bearings. If you decide to build the 10 bolt, myself or others will be interested in buying the rear or at least the posi and gears. Good luck.
Last edited by olds 307 and 403; January 5th, 2022 at 07:30 PM.
Tough call, since you need a posi and gears for the 10 bolt, it will be slightly stronger. Cost is a tough one, depends if the clutches are toast, probably a wash between the two. That 3.08 12 bolt Type O posi will work with the aftermarket 3.42 and 3.90 gears, a huge savings over the aftermarket ones. The factory posi does not fit 71/72 Moser bolt axles, the aftermarket posi does. The 70 has stronger oiled bearing bolt in axles vs a 68 or 69 sealed bearings. If you decide to build the 10 bolt, myself or others will be interested in buying the rear or at least the posi and gears. Good luck.
It's just the cover that's 12 bolt. The gear is 10 bolt.
It's just the cover that's 12 bolt. The gear is 10 bolt.
It still makes it the unique 8.5" Type O rear which only two aftermarket gear sets are available, 3.42 and 3.90, almost $500 Canadian for a ring and pinion. The good part is that 3.08/3.23 carrier is the one needed to bolt on those gears on without a spacer. Pull an axle and see if it has the sealed style bearing, which it should. Brian Trick broke down the factory axles, every change added strength. The 71/72 10 bolt cover axles are the strongest from factory but won't fit the factory posi in that 12 bolt bolt cover Type O, it has to do with the spline angle, unless an aftermarket posi is used. I am also not sure on the conversion from sealed to oil lubed bearings, if possible. Guys have twisted factory axles at the track, especially with a manual. It depends how hard you beat on it and whether you ever plan on putting on slicks or drag radials on. The earlier the axle, the easier it will break. You can easily build either rear, that 12 bolt cover Type O saves $600 easy by already having a 3.08 posi unit in it. Your call, good luck.
The 3.08 is still not the gearing I want with that 5 speed...which of the two rear ends here would be a better choice for putting the money and effort into?
If it was me I would just go get a good used 90's -2000's 1500 series truck set of 3.42 or 3.73s out of pull your self yard and set those in the Corp 8.5 that in car and just save Type O 12/10 3.08 as a spare of sell it to offset cost if you have t pay someone for the gear swap on the 2.73 8.5
The Type O should have been a little less than 8.5" its more like 8.4"
Those 8.6" gears will work great in the 10 bolt cover rear. My 94 Z71 had a 3.73 open factory, seriously on an off road package truck. I added a used Gov Lock because it was a mild power truck, not BBO powered, only SBO powered😁. Problem is the posi, probably Gov Lock and will be 30 spline. There should be plenty of used posi around, just don't buy a Gov Lock and count the splines. The Gov Lock are identified by a finger and spring governor. The later 10 bolt corporate rear currently under the car will be the strongest and has all the in between ratios and multiple style of posi available.
Those 8.6" gears will work great in the 10 bolt cover rear. My 94 Z71 had a 3.73 open factory, seriously on an off road package truck. I added a used Gov Lock because it was a mild power truck, not BBO powered, only SBO powered😁. Problem is the posi, probably Gov Lock and will be 30 spline. There should be plenty of used posi around, just don't buy a Gov Lock and count the splines. The Gov Lock are identified by a finger and spring governor. The later 10 bolt corporate rear currently under the car will be the strongest and has all the in between ratios and multiple style of posi available.
Yeah, no-way Jose on the Gov-Bomb ...
He already has the 3 series 8.5 28 spline Posi diff see his pic
,
I have found new ones on Amazon for like $279 (Yukon or similar Eaton style) includes new carrier bearings and races if needed.
Yes, he does, a sideways pic, I missed it. I thought that one was open. He got pretty lucky on both diffs, I would say. Definitely build the 8.5" under the car with the 10 bolt cover. The gear sets are everywhere and half the price. I believe both take the same replacement clutch packs, which should still be available. Get replacement parts from Monzaz on here. Good luck.
Yes, he does, a sideways pic, I missed it. I thought that one was open. He got pretty lucky on both diffs, I would say. Definitely build the 8.5" under the car with the 10 bolt cover. The gear sets are everywhere and half the price. I believe both take the same replacement clutch packs, which should still be available. Get replacement parts from Monzaz on here. Good luck.
Luck had a bit to do with it, the posi rears were part of the reason I bought both the cars they came from. I lucked out that they're "good" ones because I didn't know anything about them apart from the posi aspect.
I reckon I'll go with the one that's still in a car, although that's not the car I'll be using it in. And I'll definitely get with Monzaz with my shopping list.
The converted 8.8" Yukon units are also available for the Type O, at a higher cost. Curious of the quality of these purple spring Eaton style posi? https://www.summitracing.com/parts/yga-10301
Last edited by olds 307 and 403; January 6th, 2022 at 10:52 AM.
This appears to be a limited slip gearing as both wheels turn in the same direction. The axle stamp is SA which seems to be an open diff as installed by the factory. All the SA differentials I see in the table over at Olds FAQ are 2.78 but this one spins out to 3.08 (verified by gear stamps). The gear stamping is 13A40 GM LS 28 7, and is 8.5" 10 bolt.
Yes 3.08 (40/13=3.08)
I'd also be interested in purchasing the 671 carrier if you are interested in selling it.
Last edited by v8al; January 6th, 2022 at 11:17 AM.
The first pics, the one in the car, is an 8.2" . its a split posi case.... the 8.5" corp did not come with that style. its a cone style posi, not clutch. as far as i know there are no replacement parts for the posi .....the second rear is the Type O 12 bolt like you figured out
the 8.2" in the car will fit up to 3:23 gears. if you want more gear than that, you will need the the next carrier up.
the 12 bolt type O 3:08 will fit up to 3:23 factory gears. after market gears are available for that carrier in numerically higher ratios
Last edited by CANADIANOLDS; January 6th, 2022 at 07:49 PM.
Yeah, the cone clutch posi, once worn out are junk. Interesting on the split case, good to know. Not a good pic of the posi. At least he has two choices, the Type O 8.5" would be the better choice, the axles would be the weak link. What is the max strength of the 8.2" rear?
Another way to tell the difference between an 8.2 and an 8.5 rearend is to pull the cover and look at the bolt holding the spider gear crosspin. If it takes a 1/2-inch wrench to remove it, it’s an 8.2. If it takes a 5/16-inch wrench to remove it, it’s an 8.5.
Both rear ends have the 5/16 inch bolt. My confusion continues...
The circled bolts are the game changer? Not sure if it matters, but I didn't get a photo from that angle of the one with the 12 bolt cover...which is now under a foot of snow.
The first pics, the one in the car, is an 8.2" . its a split posi case.... the 8.5" corp did not come with that style. its a cone style posi, not clutch. as far as i know there are no replacement parts for the posi .....the second rear is the Type O 12 bolt like you figured out
the 8.2" in the car will fit up to 3:23 gears. if you want more gear than that, you will need the the next carrier up.
the 12 bolt type O 3:08 will fit up to 3:23 factory gears. after market gears are available for that carrier in numerically higher ratios
It is not a 8.2 see lugs at bottom corners and Auburn makes split case posi like that for the 8.5 that why it has coil springs instead of a flat S spring like the TYPE O ..
It doesn't seem to be the aftermarket Auburn Pro case, which are smooth. I did find this pic of the regular Auburn, which is split.
If it is an 8.5", then someone added a non rebuildable Auburn in it. Take it apart and check the cones for cracks, supposedly a common issue. I haven't heard of castrosphoic failure with the Auburn but this is a 455 with a stick. People either like or hate the Auburn. I believe it just acts like an open diff when worn out. Great thread.
I have had mixed experience with oem Auburns and aftermarket new ones
The 90s B Body Impala Caprice Roadmasters etc come with a 30 spline 8.5 Auburn and can verify ones in a 275,000 mile POS car still did posi burnouts in a heavy wagon but also have installed new older one in our C10 and it failed quickly but maybe it was wrong additive I added since it was not Auburn labeled specific one ? Installed a new one in a friends truck years ago and it was fine. Prob had the supplied correct additive though,,,, the 3rd gen F body 7.5 cars have em and they seem to work for decades and we still use some original Pont 8.2 ones from 60s and well they still work... go figure
Last edited by GEARMAN69; January 7th, 2022 at 05:29 AM.
It is not a 8.2 see lugs at bottom corners and Auburn makes split case posi like that for the 8.5 that why it has coil springs instead of a flat S spring like the TYPE O ..
So the corporate 8.5” came with a split case from the factory?
what were they installed in and what years and what’s the casting number on them?
the 8.2 had springs.
Last edited by CANADIANOLDS; January 7th, 2022 at 11:01 AM.
If it was me I would just go get a good used 90's -2000's 1500 series truck set of 3.42 or 3.73s out of pull your self yard and set those in the Corp 8.5 that in car and just save Type O 12/10 3.08 as a spare of sell it to offset cost if you have t pay someone for the gear swap on the 2.73 8.5
The Type O should have been a little less than 8.5" its more like 8.4"
i don’t know what gears you have that measure 8.4”?
I have three factory sets here, 3:08 , 3:23 and 3:91
all are 8.5” exactly, not even 8.499 or 8.501
i have a set of aftermarket Richmond 3:91 and they are 8.595”
Last edited by CANADIANOLDS; January 7th, 2022 at 11:33 AM.
FYI, the cone type posi can be rebuilt. My brother-in-law had the original from his '68 GTO rebuilt back in the 80s. The machine shop took a bit off the end of the cones (and maybe the bottoms of the wells - I don't remember) so they went deeper into the wells and would then grab again. It worked well after the rebuild.
FYI, the cone type posi can be rebuilt. My brother-in-law had the original from his '68 GTO rebuilt back in the 80s. The machine shop took a bit off the end of the cones (and maybe the bottoms of the wells - I don't remember) so they went deeper into the wells and would then grab again. It worked well after the rebuild.
yep, I’ve done it too. Just have to compensate to get the spring pressure back to where it should be
but if the case has been eaten away you can’t fix that.
January 3rd, 2022, 09:42 PM
