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79 - Rear end differential cover cracked

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Old 08-11-2018, 03:52 PM
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Lagonia
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Default 79 - Rear end differential cover cracked

I was leaking a lot of fluid and it seemed to have been getting worse. I thought it was a gasket and was trying to postpone this job. I finally put it up in the lift, removed the spare tire carrier and inspected - differential cover is cracked. Son of uh......

The whole rear end needs a serious refresh so I may take this opportunity and just start working on this big job. Would welcome any words of wisdom from those who tackled this job.

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Old 08-11-2018, 04:09 PM
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It is pretty easy to replace the cover. All these years and I never broke my rear end cover. This last rear end replacement I went for a HD rear end cover. They aren't that much more than the standard. It uses much bigger diameter spring bolts and a HD spring plate. You should also look into the aluminum disks that hold the diff x-member in place.

This is the stock cover as I was starting to remove the diff.


Last edited by gkull; 08-11-2018 at 04:13 PM.
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Old 08-11-2018, 04:49 PM
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Muskegon Brake sells the H.D. cover for just over $100. Then you will need the gasket, two bottles of GM Lube and one small bottle of GM Posi modifier.
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Old 08-11-2018, 04:59 PM
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The gasket isn't necessary. Just black permatex. It makes it easier to clean up the next time you have it apart.
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Old 08-11-2018, 05:02 PM
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Exhaust is out - next is the rear spring set - it is old, soaked with differential fluid and the laminates between the steel plates is half gone. May be a new spring set....
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Old 08-11-2018, 05:53 PM
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Safely removing the rear spring set:




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Old 08-11-2018, 08:55 PM
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What a coincidence. I just replaced my cracked differential cover today. Took me about 4 hours. As far as advice, disconnecting the strut rods from the center bracket makes it a ton easier getting the half shafts off and reinstalled on the diff since it gives the arms lateral movement.


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Old 08-12-2018, 10:36 AM
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Originally Posted by Cool95vette View Post
What a coincidence. I just replaced my cracked differential cover today. Took me about 4 hours. As far as advice, disconnecting the strut rods from the center bracket makes it a ton easier getting the half shafts off and reinstalled on the diff since it gives the arms lateral movement.
Nice! The rear end of my Vette is in a sad state - this is the excuse I was looking for to rebuild the trailing arms, fresh bearings, bushings etc.
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Old 08-12-2018, 10:54 AM
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Next step, remove driveshaft and pumpkin
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Old 08-12-2018, 12:59 PM
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I was really lucky with the differential crossmember removal this morning - two bolts and a gentle tug with my 2' pry bar and it came off.
The pumpkin is on the engine stand - time to open it up and see what kinds of surprises await me!!!




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Old 08-12-2018, 01:23 PM
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Default Differential is toast

The differential is toast just like I suspected. What would cause such a catastrophic failure?????

I am open to suggestions as to my options - I have never rebuilt a differential but I do have the knack and tools for something like that. I did rebuild an engine and transfer case.

Where is the best place to source parts? If I decide to just go with a rebuilt unit, what are your recommendations for vendors?

Thank you!




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Old 08-12-2018, 06:56 PM
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My differential was rebuilt last year, all my parts came from Tom's Differentials. He also has some videos on Youtube. This is his web site address https://www.tomsdifferentials.com
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Old 08-12-2018, 09:30 PM
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Tom passed away last January. His wife JoAnn is still running the business and will have the parts you need. The ring gear bolts backed out, locked up and sheared. Common enough issue with the 77-79's. If the posi case isn't too bad you might be able to reuse it. Use Loctite #271 on the new bolts if you rebuild it. Looks like you have a nice shop there so if you have the time and tools you might be able to rebuild it there. All depends on what your plans are for the car, street use or hard use.
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Old 08-13-2018, 08:00 AM
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Originally Posted by GTR1999 View Post
Tom passed away last January. His wife JoAnn is still running the business and will have the parts you need. The ring gear bolts backed out, locked up and sheared. Common enough issue with the 77-79's. If the posi case isn't too bad you might be able to reuse it. Use Loctite #271 on the new bolts if you rebuild it. Looks like you have a nice shop there so if you have the time and tools you might be able to rebuild it there. All depends on what your plans are for the car, street use or hard use.
This car will be used for street use only - sedate cruising. I am planning on rebuilding the differential myself. This would be the first time for me. Any advice will be much appreciated. I removed the carrier from the case and cleaned the case - it does not actually look that bad. You are correct, one of the ring gears backed out and it sheared off 6 out of the remaining 9 bolt heads. In addition to that, it took one of the carrier's bearings out. I will post some more pictures tonight.
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Old 08-13-2018, 09:48 AM
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While the problem you have is not uncommon, breaking the cover is. Is there an impact spot on the inside of the cover from a bolt shooting into it? Covers break at the ears from improper spring loading or a heavy hit.

For your application you can build a strong diff at home without a lot of machine work, if you have access to a lathe, mill, or surface grinder even better.

If you have questions on rebuilding the diff you can contact me directly and I will go over it with you.
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Old 08-13-2018, 07:06 PM
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Originally Posted by GTR1999 View Post
While the problem you have is not uncommon, breaking the cover is. Is there an impact spot on the inside of the cover from a bolt shooting into it? Covers break at the ears from improper spring loading or a heavy hit.
For your application you can build a strong diff at home without a lot of machine work, if you have access to a lathe, mill, or surface grinder even better.
If you have questions on rebuilding the diff you can contact me directly and I will go over it with you.
Thanks GT1999, I will take you up on your offer once I do my homework, am able to converse intelligently on this topic and I start back in the assembly process. Unfortunately, I do not have access to a lathe or mill (a mill is in my short list, however).
There is a scrape in the insight of the cover corresponding to the crack plus an internal section of the cover broke off (ear?). I shall post some pictures shortly..
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Old 08-13-2018, 07:23 PM
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Default Is the case salvageable?

Posting pictures below of the empty diff case. I had it cleaned in an aqueous hot bath (industrial parts washer). There are definitely some issues but with the judicial use of a grinding store to get rid of any sharp edges I may be able to reuse. The cover is toast and will post pictures next



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Old 08-13-2018, 07:33 PM
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Default Differential cover

A couple of pictures showing the damage on the cover



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Old 08-13-2018, 08:01 PM
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I could not find any issues with the ring gear itself (ignore the bolt head stuck between the carrier and the ring ). The pinion gear had a couple of issues shown with red arrows. The carrier itself had a couple of "bruises" but nothing serious (or so I think)

I will be disassembling the carrier next to take a look at the clutch pack and planetary gears.



One bolt got loose and caused 6 out of the remaining bolt heads to be sheared off.


Could not find anything wrong. Ran my bear fingers on the edges of the teeth and detected no problems


Chipped teeth on the pinion gear


Chipped tooth on the pinion gear


Surface "bruises". Note that the bearing on top was destroyed

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Old 08-13-2018, 10:00 PM
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Your pictures didn't show on my screen.

I can see them now, that took a good hit inside. The left hand damage is common and usually can be dressed with a die grinder and some time. Look for cracking at the pad holes. If you find any replace the housing, in fact just get a good core and rebuild it. The RH pad damage is not common but really must have locked up and broke loose like a cannon shot. Check the posi case close for cracks, check the ring gear holes for being egg shaped- if found again replace the part.

The clutches used in these were not good and should not be reused, the spiders were 10-17's and were good. Mark the position of the pinion to side gears so they go back in the same way in this case or another. If there are no chips in them they are probably good- but check them.

The axles, if original, are soft faced which is no good. If they are not worn but just soft they can be hardened and reused but that rarely is the case with 73-79 axles.

Do not reuse the ring & pinion. They took an impact and if there is a fine crack in the tooth base you don't want them coming apart again.

Market value on a complete used 73 -79 diff core, out of the car is $300- $400 that's it, sorry those asking more for a core either don't know or care. When you go looking for correct dated and stamped cores the price skyrockets, only because there is a buyer looking. Rarely does this apply to 73-79's due to lower resale value of the car. Rebuilders pay less then that. That might be your best option given what I see. If you want to use yours for training go for it, back in the day we would build what we had or just bolt in a core and go.

Last edited by GTR1999; 08-13-2018 at 10:38 PM.
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