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Discussion Starter #1 (Edited)
I'm planning on a very detailed Differential thread but I wanted to pass along what I found on the 68 diff I opened up today for one of my forum buddies.
The history on this as I know it was it was rebuilt by a vette guy or vette shop 10-15 years ago. There was no apparent problem but the owner wanted to check things out and dropped it out and opened it up. He found the pinion hit the posi case and then decided to have me take a look.





For those of you following the posi tuning post this is an early posi case with the square window.

The first thing I noticed was no witness marks on the caps. I will have to test fit the caps on the new races to see if they are mixed up or require fitting for a .001 rock fit.

The case was chewed up pretty good and then I found a crack. Too bad but this one is NG now.



With the case out you can see where the pinion was kissing it.



This is an early 65 diff not an original 68. The design of the pinion was carried over from the 63-64 design only with 30 splines. This was a weak design and many pinions sheared at the bottom of the small crush sleeve. The posi case didn't cause the impact the pinion moved and hit it.



The splines appeared to be turned down as well. The date code is 1965 336 ratio.



The yokes had .030 endplay and the owner figured he was going to need new yokes. I suppose I could have stuck him with new yokes but that is not how I work so I measured them. If you see the chamfer on the end and the distance from the face to the snap ring is .187 +/- .005" then they are still good. These were .190" no wear at all and I'll reuse them.




The point of this thread is for you guys to learn as much as you can about this stuff so if you farm it out you can ask questions. The one good thing is the cracked posi was found before it turned into metal salad.




 

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Great post!!:thumbsup:
DAMN, Gary, I ain't seen one like that since my 8.25 Mopar posi rear in my 3/4 ton van tossed the cross shaft into the pinion.....

AFAIK, there is only one thing that will do that, lock it up instantly so tight I slid that truck 50' both rears sliding....nuttin locks them up like that cept ONE thing.....took the cover plate off saw the carnage.....it's the only one like that bad destruction in some near 50 years of personal observations....

great pix....

NOW for a question....when I do these things, I just know enuff to set the pattern, when it looks like your avatar there, I consider it done....

BUT, I really have no clue how to perzactly set the pinion depth per specs....

what do you measure off of?? do you REALLY NEED all the test setups and shafts of differant diameters, and so forth as shown in the shop manuals??

cant it be measured off a straight edge layed across where the caps bolt to??

and just exactly where are we measuring FROM and TO??? center of the pinion?? what size tool?? pinpoint?? pinion face...they somewhat irregular from what I have found....so I have given up completely on that and gone for the pattern white grease, and some drag on the axel shafts, and crank the pinion...backlash is simple,.....I move the pinion in and out to get the pattern in the center,....and keep moving the ring gear so it's concentric the same all over....dunno what else to do...

been fairly successful so far, some are noisey some are quiet, some become quieter after some miles....

Advice other than what's in the shop manuals??

other than quit doing it....

:smack :smack :cheers:
 

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Discussion Starter #4
You got a lot of questions there Gene. First off I only work on vettes,63-79's. Although the procedures are similar for other GM diff's.

I will address most of your questions in the planned picture post on diff's. I may start on it tonight. It will be the largest one of my posts due to all the pictures and procedures.

The bearing caps are machined in place and need to be correctly marked and installed back to their respective sides.

You can not reference the pinion depth from the pads as they can be off as much as .050". GM used a few tools that will set the pinion depth but I go by pattern. I get the backlash close and snug then check the pattern. It is almost impossible to get the pattern back to back but every set is different.
I use a surface grinder doing this job to dial in the shims for the backlash I want.

You want the pattern to be equal in length between the root and flank of the teeth. ( top to bottom) like in my avatar.
 

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gary are you saying the damage was caused by a too long crush sleeve? That would mean the ebarings are not loaded and loose allowing the pinon to move forwards and backwards and possibly even cock in the bore. If that isn't the case, and the bearings were tight how can it cause the pinion to hit the case, that's only possible if the pinion is shimmed up too much bet then you have to shim the carrier/ring gear over to the drivers side to even get the 2 to mesh somewhat properly.
 

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Discussion Starter #6 (Edited)
The pinion hit the case. Could have been after being serviced.
 

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Gary, I suppose it depends on the use, obviously...but here is a literal truth story....

Mopar 440 engine 321 posi rear... 9.25 inch Auburn cone diff.....

when I got it for the replacement of my seriously grenaded 8.25 inch van rear, same old brown thing I drive today...for some 15 years now....

I took that 9.25 apart and found it was nothing but short of blowing up...the pinion was SO loose in there it was joking, no wonder it howled like a banshee...front bearing, the works....all I did was replace every bearing, as they were all ground up,...set the pattern with original shims, and bolted it back together....it's under there today about 150K miles later....some of it loaded to the gille tail on the snubs, light on the sky...serious...
but then again it's only a 318 automatic, truck dead empty is 4400 lbs...so it's hardly a hotrod....but I am amazed how much a diffy can take in the use/abuse department....but MY suspect is that.....IF everything is just not perzactly kosher, like a really high performance vechicle needs to be....

it's TOAST....
 

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Discussion Starter #8
To add to this thread is a good example of a 71 diff I had in. This diff is extremly clean, from CA, but it locked up and chewed up a tooth. When I took a look inside I found some interesting things. First it was worked on in the past but who ever did this job makes "bubba" look like a super-genius!
When I got to work on it this is what I found:

Broken tooth


Ring gear bolts were rounded off from smashing into the housing.


The original RG bolts were gone, replaced with crap and mixed up. No lockwashers on 1/2 of them and flanged head on the othe 1/2. Real bad.



With the posi out you can see the damage to the LH side of the cap pad and housing- common when a Rg bolt backs out.









Here it is after some work, it took some time but this housing is still usable and blended in nicely.





All the internals were junk,nothing was any good. Now it's going to have a very nice new polished case.



Here is what fell out when I rolled it over, the RG tooth and one of the posi washers. Who ever had the posi apart didn't get the washer in place when installing the spiders and it got a bit chewed up.





Just something to check when you take a look inside your original, used bought, or rebuilt differential.
 

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Discussion Starter #9
I picked up a swap meet special this weekend. It is a 79 diff and I only really wanted the housing to build a 12 or super 10 bolt out of. Last week a corvette shop accused me of scare tactics on another forum. That, of course, is total BS. My intentions are only share the information I have with others, even some vette shops!

I hear a lot about guys buying used diff's and bolting them in and going on their way. I wish them the best, the money they saved on a quality rebuild can be used elsewhere or to pay for the flat bed in the future. Buying a used vette diff is risky unless you know the car it came out of or intend to rebuild it.

Here is a good example.This diff was "rebuilt" and came out of a running 79 with "no issues".

You can be the judge, the housing is good so I broke even on the buy, someone who bought something like this one to bolt in would have taken a hit.

Not many picture because most of this you can find in my other diff threads. The side yokes are shot, not a surprise. The rebuilder made a cover gasket which was leaking. The pinion was binding a little bit which turned out to be the bearings. The bearings were a mix of Timken and imports- pinon over crushed. The stock case shims were gone,not a surprise with a rebuild, but instead of nice machined steel shims they used a mix of 63-66 shims and homemade. The pinion hit the posi case,the cross shaft bolt was sheared which broke the posi case hole out. I will toss out the posi, R&P, bolts, core the yokes, and use the housing and pinion yoke. Given the parts required to rebuild this one it was not a good deal. No scare tactics here just a bad rebuilt used diff from a swap meet.

Shims found in diff


Stock cast shim


Bolts used






Broken case hole and pinion hit



 

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Nice post Gary:thumbsup: The guy I work with rebuilds diff's and is very precise and picky when he does one. Me and him rebuilt a Dodge rear end and the owner put a pinion seal and some other stuff in it , but it was making noises and acting up and when we pulled the pinion out the bearing had exploded and was lying under the pinion which wiped it and ring gear out. Usually the ones I help him rebuild always have broken parts due to poor rebuilds, crappy parts, or lack of maintenence. Keep up the good work Gary:cheers:
 

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Discussion Starter #11
Here are some more pictures on a 75 Diff I just got in. This came out of junk car, no surprise there. The axles were so worn they hit the housing machining the 1/8 boss down into the seal. The axle faces were worn down past the snap rings so they are junk now. The axle u-bolts cut into the ears on the housing. I don't know how good the posi is either. Most of this is going into the scrap barrel.


















 

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Gary,

What a great, informative post! Even though I had set up my share of 12 bolt Chevy rear ends in years gone by, I passed this part of the build over to a guy I worked with at the local Chevy dealership well over 30 years ago. I'm glad I did, as this is an area if your not really fluent with, it's probably best to leave to the experts. I felt the same way about my trailing arm rebuild, and let a guy who's done 1000's of them handle the task. Too much trouble to have to re-do something than it is to do it right the first time. What an awesum thread!:partyon:
 

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These threads are all amazing. SO much info and great photos. Thanks Gary for taking the time to post and update.
 
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