Jayson77

Looking for information on what problems commonly make differentials unrepairable or unacceptable cores.

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ratflinger

Cracked or exploded posi case is probably #1, ring gear bolt backing out and causing an explosion. Otherwise it should be repairable.

69L88

Big difference between unrepairable vs unacceptable. Add in undesireable as another category.

While one can apply some general categorization of least to most, the fact is that any unit is subject to having been “rebuilt”, some multiple times, so its a Forrest Gump Box of Chocolates exercise when buying.

63-64s are very undesireable DANA units and extremely tough to find parts for them so they fit, in one way or another, into each of the three.

65-66 cases are narrower than the 67-79 so can’t use the speed shims. Not a show stopper but more of a PITA.

There are three “series” of posi cases. The most desireable are the 3rd series for building a differential that can handle more power.

18 tooth spiders were the norm until 72, I believe, when stronger 17 tooth gears were introduced. These gears were all forged but what is available new today is “near net forged”.

Posi clutch plates come in 3 types, the original solid steels, a serrated style, and finally fiber coated. In this instance, the earliest style is the best.

USA made gears are the best but, sadly, are no longer available. Good, matched, OE GM gears are good but hard to find with confidence they are original match pairs.

Knowing the bearing caps are original to the case is critical but, again, with the chances high of being rebuilt, all bets are off here.

Stub axles are another crap shoot. Starting in the early 70s, GM QC went south and most axles were not properly heat treated, resulting in badly worn tips. You can easily measure them and there are some who repair them but you need to understand the pros and cons of each of the repair schemes.

if you can find a differential that was built by Tom’s Differentials, that’s top shelf. Also, one rebuilt by Gary Ramadei. Most everyone else is a commercial rebuilder and you are gambling with those.

Bottom line, you pays your money and you takes your chances.

leigh1322

I would use some carb cleaner and look for cracks, in the corners of the big window, especially the lower right. Kind of hiding behind the gear.
I found 4 cracked used ones out of five. Not good odds.
Also look for cracks and especially an oblong hole where the center pin goes.
Either of those will cause the posi housing to be a doorstop.
One of mine had a loose bolt. Visible impact marks where the bolt jammed.
Housing gear flange was no longer true, but we checked it and trued it at the machine shop.

zwede

As stub axles wear the flange moves closer to the side of the diff case. Eventually the flange cuts into the diff case and makes it impossible to install a new stub axle seal (new one falls out because material is missing). That will make the core unacceptable.

GTR1999

I think we addressed this awhile back. The last I heard you were going to rebuild your own axles and the diff? Did you do that or as it sounds maybe bought a vendor built diff and they rejected your old core, keeping about a $500 deposit? Maybe like many of us you just didn't get to it?

Wear on Diff case - CorvetteForum - Chevrolet Corvette Forum Discussion

Jayson77

QUOTE

No, I haven't had a core rejected. No I haven't bought a vendor diff.

I was looking for the old post, for info, that you attached but I couldn't figure out how to get to. Thanks for that.

I do still plan on doing my own axles, I'm just trying to find time.

leigh1322

Wow!

In that old thread GKull said his 79 L82 diff wore all the way thru the snap rings in 50k miles. And I remember he said he bought that car new. That is at least .185" + wear on the axle stubs, or more.

That data point proves how much GM's axle hardening had changed over the years, and how bad it had gotten.

My 72 BB must have had much harder axle stubs, because I barely measured any wear, they only wore .005" in 90k miles! They were till .185" above the snap rings. And Gary said they were still good to re-use. We even checked the rockwell hardness of the wear tip at RC52 IIRC.

GKulls diff had 40x as much wear! Due to soft metal.

leigh1322

Jayson;
With that much wear you need new axle stubs. That's a given.
You may as well look for those and get them now. It could be hard to find good ones.
Then you may as well take it out and open it up yourself, to see what other issues you may have.
Then you can decide how to tackle it.
If if you send it out, you will need to take it out, and somewhere the car can sit for a while with no IRS.

Important to make note of now:
Is it make any noises?
Whining, grumbling, etc?
If it is quiet, you may be better off keeping the GM gears.
If it is quiet, I would not drive the car until repaired, if you want to possibly save the gears.

But I would go ahead and order Gary's preferred bearings and seals now.
IIRC you can find that in my diff thread:
https://www.corvetteforum.com/forums...iff-build.html

Just DO NOT take it apart before you measure the gear pattern and the backlash if you are planning on re-using the gears!