Differential Rebuild

[XL Automation]

Well, this is a good question for GTR1999 and I almost just PM'd directly, but thought that others might find this useful.

Thanks to Gary for talking to me the other day about my 68 diff rebuild. I was concerned that my cross shaft bores were slightly large at 0.751 or so, and that question turned into a long conversation

Anyway I took his advice regarding the poor man's brake cleaner magnaflux and I found a 1" long hairline crack in the posi case. I already have a new gear set, bearing kit, and clutches on the way from Tom's. This is a driver, so while I want quality, I don't need to do any unnecessary upgrades.

I have a few choices here:
1) Find a used differential, tear it down and use the posi case from it. Of course the danger is that I will end up with another damaged one.
2) Find a used, cleaned and checked out posi case from a supplier (suggestions welcome). Might want to upgrade spiders to 17/10 from the 18/??.
3) Purchase a rebuilt complete posi unit. Disadvantage is cost, unknown rebuild quality, I already have new clutch packs on the way, and I miss out on learning how to rebuild a posi.
4) Purchase a new Eaton or Auburn complete posi case. Costly and I don't know if these will fit my carrier without any mods or needing new axle shafts.


Any suggestions are welcome. I'm leaning towards #2. This is my first diff rebuilt and although I feel competent, it is all new to me.

Michael

[gtr1999]

Hi Mike
Nice speaking with you the other day as well. Finding a cracked posi in an original '65-68 and even 69's is not uncommon. I'd say 60-80% of the originals I check are cracked. it was an attempt at improving the terrible Dana's used in the 63-64's but really not much of an improvement.
The posi case design was the flaw, the 70-79 cases are much better. ALTHOUGH I just inspected a "vendor" rebuild diff with -0- miles on it and it had a 2" crack in the posi. Kind of hard to understand how a rebuilder could have "missed" that. Stevie Wonder could see it.

So you have a 68 case and 10-18 spiders, I would deposit them in the recycle barrel, the 18's can be used if they are not worn out but the 10-17's are the better choice.

So now what do you do?
#1 really isn't a bad idea, a diff out of a parts car is worth about $350-$400 tops, with cover and axles. Paying more for a used unknown one is not advised. Yes you could end up with another bad case but there are a LOT of rubber bumper 308 diffs from automatics out there and I never paid more then $400 for one- complete. All 71-79's used the 10-17's so you only need the posi. The axles will be cores, the housing in nice shape, blasted, POR15 and top coated is worth $300. The R&P I trash but some sell. The pinion yoke is a $50 part. You will need to use the 1310 yoke that your diff probably has on it.

#2- Hard to find, vendors will ask $250 or more for a bare housing, you can buy them new from GM. List is about $425 or so now. I see junk being sold all the time so I would not go this route unless you can see it first, just like with the option above.

#3- Buying a built used posi from a non custom builder is not worth it. You will get a stock setup at best and the weak areas will not be addressed. You have the new parts go with a bare case and build it right.

#4-New load posi's will bolt into your carrier. This is the route the common rebuilders use all the time, saves them time and they mark them up pretty good. Look up the Diff test thread I have here and decide for yourself. I wouldn't use one at 1/2 the price.


My choice is simple, buy a bare case, polish it and tune it just like Tom shows in his You-Tube video. Use the solid steels, forget the fiber nonsense and the springs/plates. If you follow my threads and Tom's video it will be the way to go.

Doing it yourself will save you the most money, just do it right.

[XL Automation]

Well, I'm working on the pattern now. I have a backlash between 5 and 7 thou. I put the same thickness shim in the pinion as original (.030). Here's the pattern (on the far right -other marks are from previous test fittings with the original bearings). The pattern looked good with the old bearings and I had a good BL, but seems to have moved a tiny bit to the toe with the new bearings installed. The pinion is just under your recommended in-lbs, but it is close. I figured I'd get it perfect on the last tightening with the new nut and crush thingy.

If this seems a little too much to to the toe, do I take a few thousandths out of the pinion shim?

Recommendations are appreciated.



Thanks,
Michael

[gtr1999]

Pattern is too deep, using the original shim won't work with aftermarket gears. Go to a 026 shim, keep the lash at 5-7 and recheck it. Look at your pattern, the root length is longer then the crown length, you want them even more so then centered on the tooth.

I see you are using a stock posi setup, I recommend you polish and tune it, at least polish it. I can see the edges are rough.

[XL Automation]

Thanks a bunch, Gary!

Well, I've momentarily chickened out on spending the time polishing the posi!

How's this setup? I can't believe that .004 thinner shim can make such a huge difference in the pattern.

I noticed though, that the pattern on the coast is way off to the toe. Is this a problem?




Michael

[gtr1999]

That looks better, it is centered on the drive side, the lengths are equal. I can't see the coast. Be sure to clean the Rg & pinion every time you take it apart, the paste may need a drop or two or gear oil to mix in.

[XL Automation]

Here is the coast. As you can see (not the best photo), the pattern is well toward the toe even though the drive side is much better centered.

Maybe I can get a better pic this evening.

[gtr1999]

Clean the gears,thin the paste and make sure to load the pinion while testing. If the coast is equal in length that is where it will be best. Don't try and get a text book pattern, rarely happens. As long as the pattern length is equal you are good, play around with it a little and you will see it probably won't get better but will start to move off center.

[gtr1999]

So what is the status of this job, are you out blowing off the tires yet?