Without taking this too far off course, what I pictured is not, repeat not, uncommon- and from more then one place.
Some examples and there are plenty of more. Some of the people leave here and set up shop elsewhere like FB or other online forums.
1- Steering boxes sold with bad gears
2- Trailing arms with excessive endplay or runout, cheap axles.
3-Differential sold as a certain build when clearly it is not
4- Differentials rebuilt with bad or cheap parts
5 -"guru"s taking money for services not provided or for lousy work

There are plenty more but that is not the intention here and I don't want this to be off the OP's path so I will retire from posting on this thread any further.

My point was made to answer your direct question. Those that don't due their due diligence will find out in time just what they got or thought they got, as some have already documented here in the past.

Good luck is about all I can say.

 

Well Gary I know exactly what you are saying.
I do not know anyone else who I would take differential advice from.

I try to do my "due dilegence" and do as much as I can myself. Then I know it is fixed correctly. Be it my daily driver or my hobby car.
I tend to only go to the shop when A) it is above my skill/tool level, or B) I don't have the time. It is hard enough to get good work done right the first time even on a current car, much less one that has been out of circulation for 40 years and many of those skills, and those people with that knowledge, have just vanished with the passing of time. Get a Carburetor "tuned up" at the local gas station ? Are you kidding me? Do you mean at the WAWA or the Costco gas pumps? LOL Even the dealership needs 3-4 times to get it right these days.

There are very few people I trust with my families cars. There are actually more more people on this forum that I trust for their respective skill, knowledge and expertise, and these old cars would be hard to keep going with out them.
So my hats off to those of you who I trust almost implicity:

GTR1999 Jebby Allvettes4me Gordonm Lars bkbroiler 69L88 JBL82 Willcox 70sVetteguy flyboy1958 Rescue Rogers BigBird
AJRotham and many others

Your respective knowledge and willingness to help is what keeps this hobby fun, for those of us still learning. If only we could all get together for some R&R!

 

Ok Diff #2 autopsy.
Looks like I am still in the market for a good used posi unit!


This pair of yokes was severely worn - past the clips. They pulled right out of the case with no disassembly. GTR199 says these are the years (77) for the soft faced yokes.


Someone put GM 3.70 gears in this 3.36 1977 diff, way back in 1979 or so!


Posi case pin hole is pounded severely oblong, bolt has actually broken, pin is floating and also bent



Missing ring gear bolt as well.

So I am just guessing this 77 was a stick car and someone beat on it pretty hard. With all the metal that had to be floating around in there due to the worn yokes, missing c-clips, the missing RG bolt, and the floating spider pin, I am not sure even the gears would be any good.

I'll check them out, but I still need a posi case.

 

Leigh,

I had no idea that the unit had a problem. Send me your address and I will send you a check for it or you can check you one of the other two units it have.

Bob K.

 

If anyone reading this is thinking about getting a differential rebuild I can personally attest that Gary (GTR1999) is as good as it gets.
Not everyone is as brave as those doing their own rebuild as on this thread. Hats off to your talent fellas.
But, if you want it done right and can't do it yourself Gary is the go to source. He also rebuilt my original steering box and half shafts.
Excellent communicator as well as detailed pictures. Don't know how he does all of the pictures as well as the actual work.
My differential was an original heavy duty 3:36 posi from my '69 big block. Gary upgraded it in every way and switched it out to 3:73 gears at my request.
Thanks Gary!

Best Regards,
Mark (from Florida)

Here are a few pics of mine.

 

Yes Gary does do great work. And he is willing enough to share his " fixes" so stubborn do-it-yourself-types like me can do a half-way respectable job! LOL
It's a good thing I enjoy new challenges! I for one have never done this before. And I would never have the confidence to try it with out his pictorials. I even bought a shop press just for this job!

There is great satisfaction in meeting an obstacle head-on and learning how to conquer it yourself!

 

That's the spirit.
I too enjoy a challenge and to try doing some car work that I have never done before.
But for me, taking out the steering column is a huge victory. My dash install is one thing after another.
I realize my limitations but keep trying to move forward. This car isn't going to fix itself.
Changing out valve stem seals & adjusting the valves (with the heads still on the car), installing the intake and dropping in the distributer was my most recent success this past summer. But man that was tough.
I don't see how a mechanic could do that in a day or two.
Took me about three weeks. Each cylinder kicked my butt and leaning over the fender, well after four hours I was done for the day. Back again the next day, etc.
Doing that type of work for forty years takes a physical toll no doubt.
I have much respect for mechanics, body shop & paint guys as well as excellent machinists /restoration folks such as Gary.
You Sir, are way out of my league, lol.
Best of luck

 

Well I am a school teacher by trade and a mechanic by lifelong hobby. However last summer a friend needed some help so I was a mechanic / helper / manager / guy friday at his shop for the summer. Brake jobs, ball joints, AC work, etc. Normal stuff. (SO many people have NO clue). Two other 20 year old mechanics. I got them done but I was slow as heck. Good thing I was not flat rate LOL. But when a Jag XJS convertible came in for a fuel pump, no one else wanted to touch it. It took me a week, but I got it done without a scratch! (The fuel tank comes out thru the trunk after you remove a convertible frame bulkhead. WTF!?!?!)
It felt like a comedy movie: "The 65 year old Apprentice"

 

Well I wound up with three new to me broken vette diffs. I will scavange them for parts.
All of these rears have at least one issue. The only real part I need is the posi unit.
I see two different casting numbers on the posi units. 30117 and 32297.
Is there any difference?
Is one stronger than the other?
I think the 30117 is in the 69 rear and the 32297 is in the 70's rears.
I thought Gary said the earlier cases were weaker, especially the ones with the square window.
But both of these 69 units do not have the square window.
No obvious difference that I can see compared to the 32297 70's versions, but both of them are busted.
If the 32297 is indeed stronger I will wait and hold out until I find one of them.

 

Here is a 69 and a 73 side by side. Note the additional material on the 73 inboard of the ring gear flange.

 

Thanks for that. I should have all 3 of my posi units on the bench by the end of today. Will do some comparing. I am leaning towards the 32297 (on the right above) One would logically think that the third design C3 Eaton Posi would be an improved and stronger design versus the second design. But curiously, both of my broken diffs are the third design and the good ones are the second design. Is it just Luck? or a Strength difference?

 

Impossible to positively determine. No doubt that the increased beef on the above example contributes to improved strength in that region of the casting but I don’t know if GM specified any increase in thickness in other areas of the case. As Gary (GTR1999) points out earlier in this string, GM did change the casting molds to eliminate a seam on the later units. I don’t have a set of comparative pictures of the insides.

Each case has its own unique history. Some operated behind an unmodified small block carefully driven. Others had to deal with a hopped up 427/454 whose owner cruised Woodward Avenue every weekend or, worse, his/her local drag strip. Also, being a mass production part (more than 500,000 manufactured), not everyone was flawlessly cast and machined.

You pays your money and you takes your chances but I cherry picked a 73 unit just for the posi case (and the 17 tooth spiders).

 

I know not many people have two posi units to put side by side, so I thought I would photo document.
This is a 30117 casting (left) and a 32297 casting (right)
There are differences in casting mold lines. window corner curve shape, and wall thickness.
Gary said the later one without the two casting seams "should" be stronger.
Left one is out of a Feb 1970 rear, right one is out of a 1972. Both of these had 10-17 spiders.
Gary says the 69 and earlier units have the weaker 10-18 spiders, but this 70 did not.
The 70 had the solid clutches while the 72 had the snowflakes.
As 69L88 said, it's almost impossible for me to determine which one is stronger, if either.
The 72 unit does have a smoother curve shape around the window.
But I have still seen plenty of cracked ones, including this one.

 

The casting seams are not necessarily a bad thing, provided that the transition seam is smooth. If the seam is highly pronounced, and/or has sharp edges, they should be blended. I am very careful to not remove any more material than necessary to achieve a relatively smooth surface in order to avoid wall thickness reduction. I prefer to “detail” the case vs “polish” as is the term most often used to describe the effort. The main objective is to remove any potential stress risers.

Avoid the snowflake clutches.

The 73 and later is the unit of choice.

 

That's my plan. I have another known good 73+ posi unit coming in the mail / ups.
The older units that I have here I may just clean up and practice on and sell. I have no intention of building a rear for someone else, but I will have quite a random selection of good parts available. I feel these would be better suited good for a lower TQ SBC build or a milder BBC than mine.

69L88 I am interested in your reasoning to go with the steel cap? Do you intend to drag race it?
GTR1999 feels the cap is not needed unless drag racing with slicks, so I'll go with his recommendation there. Something I'll never do is put slicks on it, and drop the clutch at 3k+ rpm. I did that a few times in my LT1 Z28 and could not believe how violent that was, but at least that car had a 12 bolt rear.

OTOH I will go with the cryo hardening of the posi case. That may help directly with the somewhat frequent cracking many of these posi units have near the window / pin.
How much it will help I have no idea. But it sounds like it could use all the help it can get.

I did not measure the slop in these spiders, but it was a lot. So tuning the posi would help reduce shock load on the spiders and that makes great sense. So that will be done.

Next step: grinding & polishing!

 

I put the steel cap in as simple insurance as I am dropping in a 500 Hp/500 lb-ft metal salad maker. I do not intend to drag race, nor abuse the car. I just want the sleep at night knowing if I tickle the throttle, I won’t be sweeping up parts. In the grand scheme of things, it is an inexpensive “good practice” and was recommended by Gary early in our discussions.

After many months of researching how to build these units, I ultimately decided I did not need to invest in cryo or rem. If I were to go racing, I’d probably do so but did not. The unit will break at its weakest point and no one can say with certainty that it will be Part A, B, C or whatever. I feel that a properly detailed posi that is tuned and balanced for street application doesn’t need cryo but that is my opinion.

 

I appreciate the discussion. It seems as if the u-joints or the outer axle stubs will be the weakest parts after beefing up the diff. That's why I plan to stick to street tires (no slicks) so they will slip before the axles break.

I have however seen numerous pictures of cracked posi units. Almost none of them had a steel cap and yet they still cracked in the center of the posi. Some of them had regular stock cap bolts and some socket head bolts, so that did not seem to be the weak point either, it was the case itself. That is my reasoning for cryo-ing the case itself. Hopefully the cryo hardening does increase either the tensile strength or it's ability to reduce micro-fractures or resist shock loading. I do not know if anyone understands very well the chemical structure changes that take place. I am a chemist and have read the techincal reports. They sound promising, but those reports don't link it to real world results very well. Many race teams use it and swear by it. The marketing hype suggests it can be as much as 6 times stronger. I'll be happy if it only helps by 50%. LOL ! Reducing crack formation starting at the micro level is the part that makes the most sense to me. I have tried cryo brake rotors on a race car before and they were significantly better for wear and heat resistance and warping. But that is not strength. The reports say you can feel the difference when you try to machine it afterwards, so it is harder. The cap screw bolts are 20% tougher, I looked them up. At $5 that's an easy upgrade. The steel cap would cost me $100-200 and I feel like I would be better off spending that money on the cryo treatment first. So that is where my head is at.

My goal is to have something else slip or break before the posi unit because that is the most difficult & expensive repair back there.

Other than that we are on very much the same wavelength with similar TQ/HP and similar rear end builds.

I too decided the $1000-2500 for a full 30/31 spline axle upgrade was not warranted by my intended street use. I'll just wait and see if those parts break, and how easily, before I go there. BKBroiler and AJRotham are both drag racing theirs with slicks, (albeit with automatics vs my stick), and so far theirs are holding up with stock axles. So their experience is why I did not go that far.

 

This has been a great thread, thanks for posting.
I hope you will continue to update it through your differential build and keep us updated with your results.
You are lucky to be able to do this work yourself and to have the space to do it in.
The rear differential is one area I felt was above my skill level, so I had Gary build a Super 10, and set up the trailing arms for my 73.
I did go with the 30/31 spline inner and outer axles option even though I won't be racing mine either.
For me it was just an insurance thing that, since I can't do it myself, I hope I won't need to have Gary do it a second time.
Greg

 

I appreciate the discussion as well.

Without going into a full depth metallurgical analysis (I have a MechE degree, not Material Science so I’m not going there!), like so many other issues, how cryo treatment affects the specific material involved and exactly how the cryo processor does his/her thing with your parts will ultimately determine how much benefit (and what kinds of benefits) will be achieved.

Before I would spend the $ on cryo, I would want to know the “deep in the weeds” details about how they are rearranging the atoms on my 50 year old casting made of AMS specification XYZ. Not saying it’s a bad thing, but I am an adherent to the mantra “trust but verify”. Probably should have been born in Missouri.

 

So true! Trust but verify!

My best friend / drag racer / machinist / engine builder has built over 5000 engines over his 40 year career and solved many racers issues with broken engines. He does not use cryo and neither do any of his racer friends. His comment was "he would just keep putting stronger parts in it til it doesn't break anymore". So in my case he would just run it til it broke, and then put a bigger rear in it, like a 12 bolt, 9 inch, etc. But factory stock means absolutely nothing to him and he would not waste much time trying to get a 50 year old "marginal" design to live.

So unless I can talk to a Nascar engine builder, I can't find a lot of experience with it. Those guys cryo entire blocks and it makes better ring seal, distorts less, has less blowby, makes more HP, and lasts longer between engine teardowns, and even then has less bore, ring and bearing wear with 2 or 3 times the hours on it. For crankshafts it makes them tougher and less likely to crack. Their success is what I am trying to leverage on.

Regular Heat treatment of Iron parts toughens them by allowing the stronger martensite crystal structure to prevail by allowing the softer austenite structure to convert into martensite. A Cryogenic hardening treatment just takes it one step further and assists in a more complete conversion of some of the remaining (softer) austenite. I found a scientific study that had test results that resulted in a increase of tensile strength of 31%, from 1900 MPa to 2500 MPa. Rockwell Hardness increased from 35 to 41. Scanning electron microscope images confirmed the change in the crystal structure with significantly fewer softer Austenite regions. Those results required a specific temperature cycle sequence to achieve those results: Austenization (at 990C) for 1 hour, cryogenic cooling (at -175C) for 24 hours, followed with a tempering period of 1 hour (at 400C). Skipping the two heating steps and just doing the cryo cooling alone produced almost no detectable change.

Here's a copy of the paper I found, as published in Materials Science in Engineering, in 2017.

What do you think?