Need help identifying rear differential
 

The original 8.2” rear end in my ‘68 GTO looks to have been replaced by an 8.5” one, just judging from the square casting protrusions on the bottom corners.

Attachment 637845

Trying to figure out what it came out of, but having a hard time finding a casting number through the standard layers of hard crud. I did find the stamping on the front of the passenger tube, looks like: CJ K-044 G. The only casting/date code I could find is not legible, it was on the underside front of the center section, on the passenger section.

Attachment 637842
Attachment 637843


From the stamping, any ideas on what this could have come from? Or other areas to check for a casting number? (I’ve scraped off several areas and no luck so far).

Doesn’t look like the mountings were moved or added, but will need to scrape off a bunch more crud to verify that.

It’s not limited slip, and drives rpm-wise like it has 3.36 or 3.42 gears (I’m going to remove the cover soon to change out the oil anyway, so will verify then).

I won’t even attempt. :)
Easy to tell when you pull the rear cover as the ring gear bolts are different size and thread. I can’t remember sizes off hand, but easy enough to look up. I have an 8.5 that all the experts claimed was an 8.2 but easy to miss visually from looking at the outside. I pulled out of a ‘72 Cutlass. I’m sure Ol Pinion Head might be able to tell.

Need the date code year to be certain.
(the 044 is Feb 13 of the year)

But the CJ could be a 1971 Firebird with 3.42 gear and locking dif.
Or 1973 A or G body with 3.42 gear and locking dif.
Or 1977 A or G body with 2.56 gear and locking dif.

And if you had my luck it would be the 2.56 gear rear.

:eek:

I THINK it was this one ( I have a few 8.5’s sitting around). Looks like an 8.2 by some standards.

I just picked up some gear oil so will pull the cover off this week, the insides don’t lie!

3.42 is what it feels like driving on the freeway, at 65 mph I’m just a little shy of 3K rpms with BFG 245/60 14’s. Definitely not 2.56, I’d be driving 90 with those lol. I’ll see what gears are in it this week.

Maybe the width end to end will shine light on whether it’s from a 2nd gen F body, or a later A/G. This could help me locate the casting number.

It does have some welding shenanigans on it after all, I can’t imagine these pointy mounts came on anything factory from GM.

Attachment 637895

Man, I hope you plan to grind that down. I’d hate to be working under there and hit my head on that!

That looks like an Olds 8.5" rear.

Popped off the cover, at least I know what gear ratio it is. 3.42’s. And aftermarket gears.

At least the teeth look great and oil was clean (never seen purple gear oil, assuming it is synthetic, especially since it doesn’t smell like GO). Somebody went through this rear end internally at some time…

Attachment 637960
Attachment 637961

Def, one of the two different versions of 8.5 A-body housings that Olds mfg for '71-72 Olds & Buick A-body's. Buick manufactured the style with the cutout rear cover AND the large totally squared off lugs on the bottom.

Since the early 90's have purchased near 350 8.5 A-body cores for performance rear builds, the Olds mfg style with the A10 axle bearings has always been the least common to pull.

The Olds axle plant, for some reason, also manufactured quite a few 8.5 A-body A9 axle bearing end housings with the same round cover housing. Am starting to pay attention to casting dates on that particular style housing, trying to determine if only manufactured towards the end of the '72 production run.

By far the Buick style center housing (2 plug welds visible outboard per side, as well as the cutout rear cover) is the most common of the '71-72 8.5 A-body housings. Inventory files notes I have 21 of the Buick style & only two of the Olds style.

Thank OPH, appreciate the info. You definitely know these.

Did Olds/Buick use these on other body styles as well in 71/72? Or could these be from a ‘73+ A-body? The upper control arm mounts are stock, but looks like the lower control arm mounts have been moved ~2 1/4” outboard to fit this ‘68.

Attachment 637976
Attachment 637977

Coil spring '73-77 A-body/'77+ B-body housings had inboard shock mounts as part of each lower control arm mtg bracket. Earlier A-body rears had outboard shock mounting. The oddest outboard shock mounts were on 8.5 A-body housings used on the housings in VistaCruiser wagons.

It looks like someone has crudely cut off that area on each of your lower control arm mtg brackets, that makes me believe it's a '73-77 style coil spring housing that the shock mounts have been modified.

The spread between their upper control arm perches on the '73-77 housings is several inches wider than on all the earlier A-body rears. I once ran across a '70 ElCamino where the frame crossmember area where the upper control arm mounts, had been cut out, & a section cut out of a '73 & later A-body had been welded in. The other fit problem with the '73-77 A-body rears is they are little over an 1" wider than the '71-72 8.5 A-body rears. With the factory drums installed, the width of the '71-72 8.5 A-body rears are 61 5/16". '68-72 10 bolt & 12 bolt "Chevelle" rears are also very close to that width with drums installed.

'75 Monte Carlo 10-bolt axle showing A-body control arm and shock mountings.

I think OPH nailed it. ‘73-‘77 A-body.

Looking at your photos Schurkey, and the ‘75 cutlass photo below, I can see how that bracket has been cut apart to separate the control arm and the shock mounting, and then welded back together on this differential to earlier 68-72 layout.

Attachment 637991

I’ll have to check the overall width again to get a more accurate measurement, but it’s about 55” to the inside of the axle flange (this diff has been converted to rear disc, so no drums to check overall width). And it’s about 7.5” between the insides of the mounted upper control arms (I don’t have another 68-72 A-body diff to compare this against so don’t know if this is a lot wider).

Here’s the measurement inside flange to inside flange of the 8.5 I pictured earlier. I also just double checked an 8.5 that is from a 71 or ‘72 Skylark, and it’s measurement is identical to this one I pictured.

Thanks. I pulled these so long ago, and can’t remember which is which. I could get these off of Craigslist for $50-75 each with open carriers and highway gears, so I stocked up figuring they would be harder to find in the future.

Thanks 67drake! Looks like what I have here, I’ll double check.

Yup, the inside flange to flange is 55 3/8”, so this rear end is ~1/2” wider. Not a problem.

Thanks again for everyone’s help, especially OPH. Much appreciated.

Royal Purple gear oil. It's synthetic.

Any chance this stuff smells like grape juice?

GM/ACDelco sells really expensive, synthetic gear oil (10-4016) that's listed on the bottle as having a "new grape scent".

https://www.amazon.com/ACDelco-10-40.../dp/B000QGOX1A

I'm just wondering if ACDelco is buying this stuff from Royal Purple.