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I was looking to upgrade the rear differential in my Delta 88 to include LDS/Posi. However, there is a ton of confusion going on here and not even the seasoned fellas at the local shop and parts stores know what to do here.
The diff, as from the factory, is 2.73:1, 10-bolt cover, with an 8.5" ring gear, and here's the kicker, the ring gear is 12 bolt, not 10.... I have tried to understand what is going on here, but no one I talk to has answers. I have never seen this setup in a GM line car, and I was hoping someone might have an idea of why this is. Is this a specific Oldsmobile thing?
I know some Canadian cars had different setups so I'm not sure if that is part of it. Perhaps someone could shed some light?
Check out the 8.875 Pontiac (10 bolt cover 12 bolt ring). I think it's probably what you have. See thread #5 in this post: 10-bolt cover with 12-bolt ring gear
My '73 455 Delta was delivered with an open corporate diff, 8.5" 10 bolt cover, 28 spline (axle), with 2:73 gears. 10 bolt carrier.
Was your Delta 88 delivered with the towing package? That may explain the heavy duty carrier.
I've upgraded mine to a Yukon DuraGrip Posi LSD with 3:23 gear set. New parts just bolted straight in.
3:23 was a factory option for our big Delta's in '73 with the towing package.
Also refurbished the entire diff at the same time.
Easy twin burnouts, was never good mileage anyway so that doesn't worry me.
Certainly woke up the 4500lb slab of iron. Best upgrade I have done.
Last edited by 73aussie455; March 3rd, 2022 at 03:26 PM.
Reason: Edit information
Hey Aussie. Thanks for the info, it's good hear you managed to find something. I do suspect I have the towing package, BUT it is still an 8.5" carrier with 12 bolts. However, according to the bill of sale from the dealership in 1973 the only extras were Convertible ($5450), 455 Engine ($204), Radio ($103), Rear Speaker ($23), White Wall Tires ($47), Clock ($23), and a Block Heater ($12), no mention of the towing package.
I may have to do as you have done, and just swap out all of its guts and change the ratio. You wouldn't happen to have a part number for the unit you ordered by chance?
I found the old boxes with part numbers see the pics below.
Do your research before you buy anything as your diff may be different than mine. My internals were a 10 bolt carrier.
As I mentioned the Yukon parts bolted straight into my diff. I bought a full rebuild kit separately from a supplier on ebay for the 10 bolt rear end and gave the parts and the car to a transmission shop, he did it with the diff in the car.
Eaton and other manufacturers no doubt have their equivalent LSD's. Yukon was the cheapest and easiest for me to get at the time. I've had no issues with mine in 5 years.
Joe is of course right, no actual towing package as such but options for towing. These included, wiring, several diff gear ratios, LSD upgrade, full twin exhaust with a L/H manifold and heavier rear suspension with springs and or air shocks I think. There may have been other options too. I know there were extra options for the police, ie, wider heavier steel wheels etc.
Last edited by 73aussie455; March 3rd, 2022 at 10:19 PM.
Thanks for the clarification, Joe. I remember reading amongst the paperwork the original owner gave me saying something about towing and "police package" but only that they were things you could order, but he opted not to on all fronts as far as I know.
Also, I appreciate the info, Aussie. I plan to be careful about ordering, but this will help to give me a starting point. I need to pop the axles out and count the splines just to be sure. From there, it should be a lot easier to to get something ordered, though I will need gear sets too, and a rebuild kit with shims. I would be happy to keep the 2.73, and reuse the gears in general, but I don't think that is going to happen. I went through a ton of grief just trying to find the correct cover gasket alone. I originally ordered a the standard 10-bolt GM gasket and rebuild kit as was recommended by my local part store, but that was last July when I did my huge order on everything, and by the time I got to it (about a month ago), I realized it was wrong and couldn't return it. It's not a huge deal though considering this is the only issue I have had since starting everything, and it is literally the last part I need to do.
Actually, I've attached a photo of the gasket I need. Maybe you can verify with your car and I'll know if we have different axles as I suspect. Note the model types on the gasket, and order sheet. Basically it is either a Buick or G-Series Van rear end. Very strange.
Last edited by Shaman; March 4th, 2022 at 09:11 AM.
The parts book says that there were three types of axles used on the B-body cars in 1973, the 8.5", the 8.875", and the 9.375".
The 9.375" was used under the Custom Cruisers (BQ and BR styles) and in anti-spin applications behind a 455 under the Deltas. This has a twelve bolt cover.
The parts book shows that the 8.5 came in either Type O or Type B. This should be ten bolt cover and ten bolt ring gear.
The 8.875" is the Pontiac axle. That is the one with a ten bolt cover and twelve bolt ring gear.
Here are the ring and pinion options.
The other thing to note is that the P-type axle is the only one that uses 7/16" x 1" bolts to hold the ring gear to the carrier. The others use 7/16" x 0.88" bolts.
Wow Joe, thanks for the information. So you are saying it should be a Pontiac axle with an 8.875" rear? Maybe this is just misunderstanding on my part, but the confusion comes in on what I am measuring here. The carrier-ring measures 8.5", and the outer edge of the ring gear measures (as best as I can tell) approximately 8.875".
The 1" bolts are a possibility as well. I haven't taken any out to verify though.
I think if you can read the casting numbers on the side of the carrier where the ring gear bolt heads are, those should definitively identify what you have.
FYI, had you posted this image with the annotations originally, this entire thread would have been cleared up in two posts. Don't expect any parts store jockey or mechanic to know anything about GM axles that aren't Chevy ten or 12 bolt units.
I do very much appreciate the help from both of you, Kenneth and Joe. I would have gladly put the annotation on earlier had I known the outer ring measurement. Unfortunately, everyone I spoke to kept saying the carrier diameter is the dimension of interest, not the outer edge of the ring. I had to go back and wrap a piece of string around the gear edge to the get the circumference and determine the diameter from there. But I am very happy to understand it now. Thank you.
FYI, had you posted this image with the annotations originally, this entire thread would have been cleared up in two posts. Don't expect any parts store jockey or mechanic to know anything about GM axles that aren't Chevy ten or 12 bolt units.
Joe good guess! Post #2 in this thread tentatively identified this rear as a 8.875 Pontiac.
I like the new photo in the footnote on your posts. Yours looks awesome.
No luck finding a posi/LSD as of yet for the rear end. The guys above were nice enough to help me sort identification of it. Best as I can find and tell there aren't any for that particular build. Even the rebuild kit is not something that is around much.
I haven't had much time to dig further into it the last couple of weeks as I've been dealing with the top on my free time. All signs point to it being easiest to do an axle swap rather than trying to find components for this particular one.
No luck finding a posi/LSD as of yet for the rear end. The guys above were nice enough to help me sort identification of it. Best as I can find and tell there aren't any for that particular build. Even the rebuild kit is not something that is around much.
Well, Joe it looks like I'd be lost without you. Thanks for the links. I envy your knowledge on this subject. While I know a lot about the car, there is still plenty I am new to.
February 23rd, 2022, 04:55 PM
