I told you I wasn’t going there! You are dredging up old memories from my Material Science class at the U of Minn in 1976. LOL. But, you clearly understand what I was referring to in my earlier post. If you do decide to go that route and find a resource you can trust, please post for all to learn.

 

Yes, I definately need to do more research. That paper is about chromium steels, aka: knife blades. I am pretty sure our posi units are cast iron, possibly nodular cast iron.
But the same basic principles should apply. A 30% increase in strength would be a terrific goal. I may just get a recommended treatment scheme from the cryo service, something that they have done for crankshafts perhaps, or engine blocks.

 

so stick it in the freezer overnight. backyard a$$hole cryo!

 

thw square wndow cases where the 63. 64 cases that would crack in half like garys pics. Those are notorioius for it.

 

Well poop!
Another cracked posi!
I have four posis now, and three are the newer "better" ones, and all three are busted. One has a banged out cross pin but the other two have the exact same crack in the exact same place!


All three 3rd design posis are busted. Only the 2nd design one seems ok.


Here's the crack in my 1000 mile posi !! I paid good money for it. (I used PB Blaster "penetrant" to find it, it soaked thru the crack.) Well it was in a 650HP road racer for 30 races for 1000 miles. But no hard starts. I am stunned it has a crack.


Identical crack on two identical casting numbers, 3rd design posi. The one on the right is out of my 100k mile 72 454 4spd. A low compression engine. The one on the left only has 1000 miles on it!

The only one I have that is not cracked is a 2nd design version with some rough looking casting seams. Spent a couple hours with a dremel grinding off the many rough edges and shaping / smoothing the window edges into smooth curves. It was actually much rougher inside than out. Especially the window corners. Wish I had taken a pic of how bad it looked to start with. (Not done yet).


It took a lot of grinding to make smooth curves in this area, on the inside. This is the good 70 housing. I am still not sure I want to use this one.


This is the same spot on the 72 and later castings. In as cast condition it certainly looks like a much better shaped casting, and will need a lot less grinding, but this exact same spot is where I have two identical cracks on these later castings.

So now what?
Go with the 70 one? Buy a new one? Find another mid 70s one?

 

Hard to tell with certainty from your pictures but the casting on the bottom picture in the area where you’ve got cracks looks very suspicious. Lots of discontinuity, voids, irregularities. My ‘73 unit has a lot of “meat” in those regions.

Sorry you’re having a run of bad luck buying trashed units. Maybe time to look at a new Eaton? Have you reached out to GTR1999 to see if he has one to sell?

 

Sorry to hear.
I wish I was closer to starting my restomod.
I’d send you the posi from my 73 and you would only have to pay me if it was crack free.

 

Latest update:
Cleaned and painted the 70 housing:
Was thinking about building two at once and selling one since I have so many diff parts:



Spent 4 or more hours grinding and polishing the 70 posi housing.
Was almost done when:


Yup that's another crack. That makes Four bad ones. Third one with a crack in the exact same spot!.
This one was really small and really hard to see. I'd say under 1/4"
And I had checked this one over really well before starting the polishing too!
(The fine polishing particles helped it show up on the polished background.)

So trying to find a good used posi has been more difficult than I ever could have imagined. GTR tells me these are stronger than the current new ones from Eaton so I am trying to persevere.

I have one more posi in the mail. This one may be 45 year old (NOS) new old stock and does not look to have ever had a gear mounted on it. It's got plenty rusty but that I can deal with it with an Evaporust soak.

Fingers crossed!

 

Just read this, you have had some bad luck no doubt. I just built 4 diff's recently including a first design postage stamp window and all were crack free. On the upside you found them before installing them as they would have imploded once the crack reached the cross shaft hole but who knows how long that will take. IF you found a read deal NOS GM/Eaton for under $600 you did well and it should be good.

 

I would ask that you start a new thread. This one is 3 pages long and in fairness to leigh1322, should not be “hijacked”.

 

No problem, Sorry leigh1322.

 

OK here is my latest candidate. New old stock. Never been in a car. How could this one have a crack? LOL Ring gear has never been mounted. Got it from a forum member who acquired a trailer full of old parts. Looks to be a late GM housing. And obviously just a little rusty. And only $200 because it looks awful ! Evaporust soak should fix that. Dissasembly and inspection beginning.

Keep your fingers crossed for me for good luck!

 

I was thinking of you today Lee. You never know with these cars and diff's I just did a Early 69 that had a carry over postage window and today did a 65 postage stamp. Now more than likely the postage stamp cases are cracked, these were not. You have 4 of the better case and 3 are cracked, go figure. Just stay with it.

Here is the posi case from the 65 for comparison.

 

OK I just spent 5 hours with grinding & polishing tools on my NOS rusty posi.
Here's the result.
What do you guys think?
I just hope it's enough.
(I tried as best as I can to emulate Gary's pics.)

 

Before & After.... Finally! (Same NOS posi unit)

Have I mentioned lately how much I like Evaporust?

 

More updates. And more "drama"
Went to see my machinists buddy last night. Good thing I did!!!
Turned out this was not an NOS unit after all.
He took one look and said this is where a ring gear bolt came loose and got ripped out of it and jammed into the housing. (Probably a dealer Warranty job) He did many of these in the 70s.

This was the "money shot" that proved his theory.

He was concerned that if the bolt got wedged in there hard enough to leave it's impression in the nodular iron housing, that the housing could be bent.
(My heart sank - Not Another Bad Unit!) And this was after the 5 hours of polishing work!

Initially I did not recognize the significance of the marks above. So do your due diligence!!!!

So we set it up in the lathe and checked the gear flange runout with a dial indicator. Yup - it was not true. It was out by 1-1/2 thousandths where the bolt left it's mark. Too much.


So 15 minutes later it was all trued up and good to go!.

Wheh! Saved by a good machinist. He removed about 5 thou so I'll have to change the shims to compensate later.

Then it was time to go to the milling machine and sink the retaining bolt for the large center cross-shaft as GTR suggests. Mine only extended past the cross-shaft by 2-3 mm to start with. It was kind of a ridiculously small amount.

All better now. Nice shiney FLAT gear flange. And my blood pressure is back to normal! LOL

Thanks to Gary Ramadei for his mentoring and advice!
And my buddy Jeff the machinist.
I could not have done this myself without both of their help!!

Who knew building a good diff is as precise and almost as hard as building an engine?

Hope you enjoy the pics from my insider's view.
BTW what this expertise "cost" me was several hours of old war stories, dinner, and staying up way past my bedtime! (No race cars in the shop last night)

Next stop: Cryo!

 

Glad to see this one worked out for you.
Looks fantastic.
Please keep us posted on the assembly.

 

Wow, I just read through the thread, amazing bad luck. I just did a 12 bolt conversion on my 72. The parts were bought from Tom's when he was still around, the original buyer went in another direction. I have a crack free posi that I had rebuilt a few thousand miles back with a 3:55 r&p.. Anyway, good to hear you have it going together now.

 

Todays update:
I received my posi unit back by UPS from Cryogenic Hardening. I have read many results online, and many top motorsports teams use it, even for entire engines, and they get good results. Basically the deep freeze process converts any remaining austenite crystal structure left over after heat treating, into martensite, which is stronger and tougher. I have no idea how much stronger it will make the posi unit, but I have seen 3 cracked diffs too many, already! LOL It was only $100. If it prevents me from having to dig into this diff again I'll be happy! Cheap insurance in my book.
Here's the contact info if anyone else wants it. Only took a week door to door.




And here is a little interesting "sound bite" I discovered. It gives new meaning to the term "singing rear" LOL

 

What was the thermal cycle they used?