73-77 Chevelle 10 bolt - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 7 (permalink) Old Apr 2nd, 19, 9:06 AM Thread Starter
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73-77 Chevelle 10 bolt

I believe that some or possibly all of the suspension mounting locations changed after the 72 models, what I would like to find out is if anyone might know if the upper control arm mounts on the rearend housing changed at this time? Any help would be appreciated.
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post #2 of 7 (permalink) Old Apr 2nd, 19, 10:27 AM
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Re: 73-77 Chevelle 10 bolt

Yes, the eyes on top of the housing are in different locations and the angle of the eyes is different.


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post #3 of 7 (permalink) Old Apr 2nd, 19, 2:00 PM
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Re: 73-77 Chevelle 10 bolt

Quote:
Originally Posted by big gear head View Post
Yes, the eyes on top of the housing are in different locations and the angle of the eyes is different.
I agree, because I have BOTH body styles (76 Cutlass, 68-70 Chevelles), and I have discovered something that few people are aware of.
For the 73-77 A-bodies there WAS 12bolt rear available. I have found a VERY, VERY few of these 73-77 style 12bolt rears on SOME, repeat, SOME El Caminos and Cutlass wagons------------and maybe some Pontiacs(?????).

The down side of these mid-70s 12bot rears is that the gear ratios were VERY LIMITED, such as 2.73-3.08!!! AND, AND, AND they were available with an Eaton posi. I have one of these 12bolts (from a 76 Cutlass wagon). The 76 12bolt that I have is a 2.73 NON-posi, BUUUUUUUUUUT, one of my boys came across a Pont 2-series posi unit that I plan to install into the 76 12bolt rear.
From what little information that I have acquired about these mid-70s 12bolts is that the 73-76 versions had 31spline axles/axle gears and the 77 versions had 28splines.
These mid-70s rears are often over looked BECAUSE the differential cover has only 10 bolt holes and 2 big notches, thus, it seems most people think they are a 10bolt rear. NOT!
Oh ya, and the mid-70s Eaton posi unit is bigger and more robust than the 65-72 style Eaton posi units.



Here is what the cover of one of these mid-70s 12bolt rears looks like. This type of rearend has the typical fill plug on the side of the cast iron case, BUUUUUUUUUUUT, I discovered that SOME of these rears have a cover with a fill plug as below. In my book, that's a plus.






Here is a comparison of a 65-72 style 2-series (NON-posi) 12bolt differential and a mid-70s 12bolt Eaton 2-series posi (right) differential. As can be seen, the mid-70s posi is noticeably bigger than the 65-72 style. Also, the mid-70s ring gears used larger, LEFT hand thread bolts.










Soooooooooooo, if you should locate one of these mid-70s 12bolt posi rears, they would be an excellent rear to have------------------if you can live with the limited, much higher gear ratios.

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Last edited by DZAUTO; Apr 2nd, 19 at 2:23 PM.
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post #4 of 7 (permalink) Old Apr 2nd, 19, 6:51 PM
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Re: 73-77 Chevelle 10 bolt

Quote:
Originally Posted by DZAUTO View Post
I agree, because I have BOTH body styles (76 Cutlass, 68-70 Chevelles), and I have discovered something that few people are aware of.
For the 73-77 A-bodies there WAS 12bolt rear available. I have found a VERY, VERY few of these 73-77 style 12bolt rears on SOME, repeat, SOME El Caminos and Cutlass wagons------------and maybe some Pontiacs(?????).

The down side of these mid-70s 12bot rears is that the gear ratios were VERY LIMITED, such as 2.73-3.08!!! AND, AND, AND they were available with an Eaton posi. I have one of these 12bolts (from a 76 Cutlass wagon). The 76 12bolt that I have is a 2.73 NON-posi, BUUUUUUUUUUT, one of my boys came across a Pont 2-series posi unit that I plan to install into the 76 12bolt rear.
From what little information that I have acquired about these mid-70s 12bolts is that the 73-76 versions had 31spline axles/axle gears and the 77 versions had 28splines.
These mid-70s rears are often over looked BECAUSE the differential cover has only 10 bolt holes and 2 big notches, thus, it seems most people think they are a 10bolt rear. NOT!
Oh ya, and the mid-70s Eaton posi unit is bigger and more robust than the 65-72 style Eaton posi units.



Here is what the cover of one of these mid-70s 12bolt rears looks like. This type of rearend has the typical fill plug on the side of the cast iron case, BUUUUUUUUUUUT, I discovered that SOME of these rears have a cover with a fill plug as below. In my book, that's a plus.






Here is a comparison of a 65-72 style 2-series (NON-posi) 12bolt differential and a mid-70s 12bolt Eaton 2-series posi (right) differential. As can be seen, the mid-70s posi is noticeably bigger than the 65-72 style. Also, the mid-70s ring gears used larger, LEFT hand thread bolts.










Soooooooooooo, if you should locate one of these mid-70s 12bolt posi rears, they would be an excellent rear to have------------------if you can live with the limited, much higher gear ratios.
Looks like an 8.5" 10-bolt especially where you mention the 7/16" left-hand thread ring gear bolts. Does it have bolt-in axles like this one?
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post #5 of 7 (permalink) Old Apr 2nd, 19, 6:54 PM
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It is, also called the GM corporate 8.5.

Quote:
Originally Posted by David Bates View Post
Quote:
Originally Posted by DZAUTO View Post
I agree, because I have BOTH body styles (76 Cutlass, 68-70 Chevelles), and I have discovered something that few people are aware of.
For the 73-77 A-bodies there WAS 12bolt rear available. I have found a VERY, VERY few of these 73-77 style 12bolt rears on SOME, repeat, SOME El Caminos and Cutlass wagons------------and maybe some Pontiacs(?????).

The down side of these mid-70s 12bot rears is that the gear ratios were VERY LIMITED, such as 2.73-3.08!!! AND, AND, AND they were available with an Eaton posi. I have one of these 12bolts (from a 76 Cutlass wagon). The 76 12bolt that I have is a 2.73 NON-posi, BUUUUUUUUUUT, one of my boys came across a Pont 2-series posi unit that I plan to install into the 76 12bolt rear.
From what little information that I have acquired about these mid-70s 12bolts is that the 73-76 versions had 31spline axles/axle gears and the 77 versions had 28splines.
These mid-70s rears are often over looked BECAUSE the differential cover has only 10 bolt holes and 2 big notches, thus, it seems most people think they are a 10bolt rear. NOT!
Oh ya, and the mid-70s Eaton posi unit is bigger and more robust than the 65-72 style Eaton posi units.



Here is what the cover of one of these mid-70s 12bolt rears looks like. This type of rearend has the typical fill plug on the side of the cast iron case, BUUUUUUUUUUUT, I discovered that SOME of these rears have a cover with a fill plug as below. In my book, that's a plus.






Here is a comparison of a 65-72 style 2-series (NON-posi) 12bolt differential and a mid-70s 12bolt Eaton 2-series posi (right) differential. As can be seen, the mid-70s posi is noticeably bigger than the 65-72 style. Also, the mid-70s ring gears used larger, LEFT hand thread bolts.










Soooooooooooo, if you should locate one of these mid-70s 12bolt posi rears, they would be an excellent rear to have------------------if you can live with the limited, much higher gear ratios.
Looks like an 8.5" 10-bolt especially where you mention the 7/16" left-hand thread ring gear bolts.

Steve
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post #6 of 7 (permalink) Old Apr 5th, 19, 8:32 AM Thread Starter
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Re: 73-77 Chevelle 10 bolt

Freddie, I really appreciate the help.
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post #7 of 7 (permalink) Old Apr 5th, 19, 11:22 PM
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Re: 73-77 Chevelle 10 bolt

Those rear ends Tom (DZAUTO) is speaking of were used on all A-body wagons (Chevelle, LeMans, Cutlass & Century). The corporate 8.75" 10-bolt. They can be found under many a Cutlass, the rare Grand Prix 455 and oddly enough, not the El Camino (even an LS4 M21). I think the center section may have been originally used on sixties Big Pontiacs through '70, then the big motored 1971-76 B-body barges & wagons.

There was a time when each GM division used their own engines & differentials.

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the more you realize there is that you don't know." - Dad


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