# Rebuilding the rear end



## Tim68 (Dec 1, 2019)

I the middle of rebuilding the rear end.
it is the original open 256 to 355 posi. I have a issue with the inner pinion bearing. The one that came with the bearings is smaller than the original. Is this because I am changing gear ratio? anyone else have tis issue? Thanks for any input.


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## 67GTONUT (10 mo ago)

Need a different Diff when going to a 3.36 and above gearset...... I dont know if that is what is causing your issue..... but it could be


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## goat671 (Apr 13, 2019)

The bearings are not specific to the ring and pinion it sounds like you purchased the wrong bearings.


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## armyadarkness (Dec 7, 2020)

Im a bit confused by the post. What are you actually doing? Replacing a 256 with a 355? It's not uncommon to have two different sized pinion bearings, on the same pinion. As has been mentioned, you do need a new carrier to go from one series to another.

All this being said, dont assume that someone didnt give you the wrong bearings, either. Mid year changes, Chinese manufacturing, and bottom-of-the-gene-pool covid staffing, all have the parts word in a mess


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## lust4speed (Jul 5, 2019)

There are three different BOP 8.2 carriers available. First covers 3.36, 3.55, 3.90 and 4.33; next covers 2.98, 3.08, and 3.23; and finally your current carrier covering 2.56 and 2.78. If you purchased a new carrier then that part is good. The strange thing is the pinion bearings for all BOP ratios are the same bearings. Many companies seem oblivious to the difference between the BOP 8.2 and the Chevy 8.2 and I see many of them listing the Chevy gearsets for the Pontiac - and there isn't a big enough hammer to blend those parts. The Chevy pinion bearing is smaller and I'm guessing they sold you the wrong parts.


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## Tim68 (Dec 1, 2019)

So I found I had the wrong inner bearing and chase. Purchased the right one from the local NAPA and have it back together.
I replaced the ring and pinion, also a new posi unit.
Preload on the pinion is 15 in. pounds and the backlash is 10-12. The backlash varied a little each time I checked it. 
This is my first time doing a rear end does this sound correct?


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## armyadarkness (Dec 7, 2020)

You'll never get two diffs the same and there's no generic spec.

I've done dozens of diffs and just did my 67 BOP, a few times, last year.

The rule of thumb is that you use the original factory/ existing shims everywhere, to start and then set the pinion preload. At that point, it should be close enough... However, on a carrier swap, the factory carrier shims are no longer going to fly, because its a new carrier.

The trick is to look at it logically... Don't overthink a diff.

The two critical points are:

That the pinion rides in the center of the ring (which is set with pinion shims).
That the side to side ring movement is not too much or too little.
Since the diff housing isnt changing, the factory pinion shims ALMOST always set the pinion depth close enough, so if you reused them, then good!

As for the side to side of the ring:

Too tight will be very obvious. There will be no movement/ lash, and itll bind.
Too loose will be just as obvious, because the ring will rotate forward and back a bit. The greater the gap, the more it'll rotate. The ideal setting is when it DOES rotate, but so little that it's barely noticeable. That means you have a paper thin gap. Shims on the left of the carrier, shrink the gap, and shims on the right, increase it. Big shims make a bigger difference and vice versa. If you add a .06 shim on the left, you must remove one on the right, and vice versa.
The marking paint is to see your pattern and verify that the pinion is riding in the center of the ring... but in my experience, if you reuse the factory pinion shims, then it will be.


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