Differential core swap

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Differential core swap

Post by SonOfTheGrim on Tue Jan 26, 2016 8:08 am

So with my malibu up on jack stands I was able to spin the wheels and found that I have an open diff. No surprise because my car was a sport model with a 305 and the only option it has is AC. I do however, have an 8.5" rear (I think). Someone can correct me but here is a picture of my diff

Sorry for the bad picture.

So my question is, rather than swapping the whole rear end for a posi from something else, since shipping is going to kill any sort of price I get for it unless I find something locally, could I not just find a posi differential core and throw into my housing? I see alot of *relatively* cheap posi units on ebay so my question is what numbers matter? I know it has to be for an 8.5 10 bolt with 28 spline axles but what series or year models or specific vehicles can drop into my housing? Or could I find some 30 spline axles with 5x4.75 bolt patterns (do those exist?) and use a 30 spline posi core?

Examples:
click
click   I know this one is just the case but just for an example.


Last edited by SonOfTheGrim on Tue Jan 26, 2016 8:24 am; edited 1 time in total
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Re: Differential core swap

Post by dynchel on Tue Jan 26, 2016 8:17 am

Pretty sure all these cars got 8.5 rearends.
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Re: Differential core swap

Post by SonOfTheGrim on Tue Jan 26, 2016 8:26 am

Sorry I hadn't finished my first post so you only got half the question.

I've heard that in 77 it was a toss up if you got an 8.5 or the new 7.5 rear.
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Re: Differential core swap

Post by Joe73 on Tue Jan 26, 2016 8:33 am

Yes, it is a toss up if you have 7.5 or 8.5 on later year g3s. Spinning the wheels is not the tell tale if.you have a posi or not. You should open it up.
And yes a bolt in posi is the way to go. Just have to get someone to set up the gears if you cant do it. No need to change the whole rear.
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Re: Differential core swap

Post by SonOfTheGrim on Tue Jan 26, 2016 3:15 pm

I thought that if you spin one wheel by hand forward and the other wheel turns backwards then that means you have an open diff. That is what I've always heard.
And by setting up you mean shimming the ring gear and diff core for clearance, right?

My questions are really about the series. More often than not I find series 3 diffs from later model trucks. Diffs from 90s suburbans (series 3) are all over eBay and K blazers. Govlocks from 4x4 trucks too. I'm not looking for a locker anyway but is there any reason these won't fit? And if 90s subs have a series 3 8.5 then do we have series 2 or 1?

I believe I remember reading that series 3 housings have bigger spaces for the bearing races or something so you have to compensate somehow.

Sorry for all these questions but there is A LOT of information out there and it gets confusing.
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Re: Differential core swap

Post by Joe73 on Tue Jan 26, 2016 3:36 pm

Tbey can spin tovether or opposite. Only way to be sure is to loom at it. Or if you have the build sheet it will show the option.
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Re: Differential core swap

Post by SonOfTheGrim on Tue Jan 26, 2016 4:41 pm

No I wish I could find a build sheet.

Something I just thought about also:
What rear end would my el Camino have? A 76 Classic edition with 350 would likely have a posi?
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Re: Differential core swap

Post by Joe73 on Tue Jan 26, 2016 5:11 pm

Nothing is likely with these g3s. Some odd combos out there. You have to check.
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Re: Differential core swap

Post by 73ss on Tue Jan 26, 2016 6:31 pm

SonOfTheGrim wrote:No I wish I could find a build sheet.

Something I just thought about also:
What rear end would my el Camino have? A 76 Classic edition with 350 would likely have a posi?

Most likely a 2.56 one wheelie peelie.

My '73 originally had bolt in axles instead of the "C" clip, which to my understanding is a BOP rear. I still have the rear. It is a 2.73 open.  My car is Canadian built which may have something to do with the odd-ball rear. Something else you gotta look out for because I don't think the carriers interchange.
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Re: Differential core swap

Post by Joe73 on Tue Jan 26, 2016 6:45 pm

If you can post up a pic of the axle tube directly behind the brake backing plate we could tell you if you have a chevy or BOP rear. With the chevy rear, the axle tube gets a little smaller in diameter right before the backing plate. With the BOP rear, the axle tube is the same diameter all the way to the backing plate.
Carriers should interchange between the two axles.
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Re: Differential core swap

Post by 73ss on Tue Jan 26, 2016 6:52 pm

Did not know that. Thanks for the info. I'll take a look the next time I'm out in the garage.

I gotta sit down one of these days and set up a photo bucket account or whatever so I can post some pics. BTW Great build thread !!
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Re: Differential core swap

Post by Joe73 on Tue Jan 26, 2016 6:55 pm

Thanks !!! No problem at all.
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Re: Differential core swap

Post by SonOfTheGrim on Wed Jan 27, 2016 7:16 am

GM 8.5" Posi Unit - 28 Spline - Chevy GMC - Eaton-Style Limited-Slip Locker NEW
Soooo for practical purposes, if I were to put this differential into my housing, is there any reason it wouldn't work? All the numbers are right and it is listed as "works in any 8.5" 10 bolt 28 spline application".

This is just an example to apply what I am finding. As usual though, there is a great lack of information specifically about 3rd gen A-bodies.
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Re: Differential core swap

Post by Joe73 on Wed Jan 27, 2016 7:32 am

Yes, that would.work. But that ad is a little too generic for my liking. No mention of brand of the unit or brand of bearings.
My opinion would be to go with an Auburn , Eaton or yukon posi. And timken , national ot koyo bearings.
The yukon unit is identical to the eaton unit and a few bucks cheaper. It gets very good reviews.
You will also need an install kit. If your doing the job yourself then grab 2 extra crush sleeves, just in case. I like and have had good results with the Ratech smart crush sleeve.
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Re: Differential core swap

Post by SonOfTheGrim on Wed Jan 27, 2016 8:36 am

Yeah I'm not looking to buy that particular one, I was just trying to make sure I have it straight and am not getting anything wrong when I do go to buy something.

It does seem that there are alot more 30 spline units available for alot cheaper, at least used ones, so I am wondering if I found some 30 spline axles that matched in length if I might have more selection. Of course that means finding those axles. Eaton diffs from suburbans and silverados seem to be common at low prices. Does anyone know the length of our axles?
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Re: Differential core swap

Post by Joe73 on Wed Jan 27, 2016 8:55 am

Look up your axles and teacement ones at motive gear. 30 spline would be stronger and axles are available. You dont want a posi out of a stock tahoe or suburban. They are gov lock units and not very reliable. Ive blown two of them up in stock trucks while towing.
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Re: Differential core swap

Post by 73ss on Wed Jan 27, 2016 8:16 pm

Joe73 wrote:If you can post up a pic of the axle tube directly behind the brake backing plate we could tell you if you have a chevy or BOP rear.  With the chevy rear, the axle tube gets a little smaller in diameter right before the backing plate.  With the BOP rear, the axle tube is the same diameter all the way to the backing plate.  
Carriers should interchange between the two axles.  

Looked at the rear today. It actually gets a bit bigger about an inch from the backing plate. I measured 3" on the tube and 3.1" before the plate. I also found the numbers but of course the stamping is not very good. I can make out 156 as the last 3 digits. The rest of the digits is a toss up.. My service manual tells who made it but I can't make out the first 2 letters.
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Re: Differential core swap

Post by Joe73 on Wed Jan 27, 2016 9:15 pm

This is what your looking for.  A smaller diameter right behind the backing plate.  This is a chevy axle.  A BOP axle will be the same diameter right up to the backing plate.



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Re: Differential core swap

Post by 73CopperMonte on Wed Jan 27, 2016 10:15 pm

73ss wrote:
SonOfTheGrim wrote:No I wish I could find a build sheet.

Something I just thought about also:
What rear end would my el Camino have? A 76 Classic edition with 350 would likely have a posi?

Most likely a 2.56 one wheelie peelie.

My '73 originally had bolt in axles instead of the "C" clip, which to my understanding is a BOP rear. I still have the rear. It is a 2.73 open.  My car is Canadian built which may have something to do with the odd-ball rear. Something else you gotta look out for because I don't think the carriers interchange.

My Arlington, TX built 73 Monte had a BOP bolt in axle, 8.5 rear and it looked to be original equipment.

I swapped it to a 95 Caprice 8.5 30 spline, rear disc rear. I think the rear cost me $100? at pick a part.

I kept the 5x5 bolt pattern and used Moog 68-78 Ford LTD lower rear control arm bushings to get the right OD/ID bushing. The b-body lower control arms uses a bigger bushing.

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Re: Differential core swap

Post by Joe73 on Wed Jan 27, 2016 11:01 pm

The pics I posted are of my 75 or 77 (forgot which year) monte rear (chevy) with 4.10s. My original stock 3.42 year is in storage and its also a chevy rear.
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Re: Differential core swap

Post by 73ss on Wed Jan 27, 2016 11:07 pm

Interesting, Mine is buldged in that same area. I do not see any welds either.

I gotta work on getting some pics up..
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Re: Differential core swap

Post by SonOfTheGrim on Thu Jan 28, 2016 1:54 am

Yes I've been reading about the Govlock, as well as the other units, and it seems like the govlock has small parts that give out when abused just slightly in the wrong way. Of course they are pretty cheap and plentiful so if you break one its easy to replace, if no other damage is done. But its still not what I'd settle on.
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Re: Differential core swap

Post by Joe73 on Thu Jan 28, 2016 6:29 am

Yes, easy to replace as in bolts rigbt in. But you have to go through checking and resetting your gear backlash again. Pretty time consuming and it has to be correct or you will have noise or worse.
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Re: Differential core swap

Post by brownbomber77 on Thu Jan 28, 2016 9:03 am

If you're not experienced setting a rear end up. I've seen shops consistently charge $400 just to set up the differential with new gears. You're best off getting a posi axle from someone or a B body rear axle and either redrilling the axles or getting axles from an 89-90 Caprice only. They have 30 spline and 4.75x5 bolt pattern.

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Re: Differential core swap

Post by 73ss on Thu Jan 28, 2016 2:49 pm

Joe73 wrote:This is what your looking for.  A smaller diameter right behind the backing plate.  This is a chevy axle.  A BOP axle will be the same diameter right up to the backing plate.




I had a look at the rear that is presently in my car now. It looks identical to the one you have pictured. I think it was from a '75 malibu. Posi, I added a richmond 3.73. This was 20 some years ago. It is a "C" clip type.

My original rear buldges right before the backing plate. It looks like the buldge houses the bearing. I think it is a bolt in axle. Reason being is it has a flange that is bolted on to the outside (Brake side) of the backing plate, much like any bolt in axle rear that I have seen. I cannot say for sure though without pulling the cover off. I tried to pull on an axle. It wasn't moving. Maybe this weekend.

Also, Good to know about the 30-spline B body axles, Did not know that. Learn something every day..
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Re: Differential core swap

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