73aussie455

Hi All
I'm looking at changing the diff ratio in my '73 Delta to give it a bit of shove off the line for a bit more fun. It only ever gets driven for cruising and fun anyway.
It has the corporate 8.5" 10 bolt rear end with 2:73 gears, I think?
See; https://classicoldsmobile.com/forums/eighty-eight/74829-73-delta88-diff-identification-blank.html

The optional ratios for a '73 Delta for the open diff was 2:73(std), 2:93, 3:23 (455 towing package with dual exhaust and 4v carb).
Also listed for Cutlass, Cutlass S, and Cutlass Supreme are G80 Posi with 3:43(std) or 3:93 ratios.

I have found a supplier here in Oz for the G80 gears so if the Posi gears fit the open diff I think I'll go with the 3:43 over the 3:23 gears.
Is this a viable option?

I will be running 22" rims with overall height of about 26 1/4". Std 15" rims and tyres look to be around 27 1/2"?
Any replies welcome

Fun71

Doesn't mater if it's posi or not, the gears will fit into the housing. All 8.5" 10 bolt rears can use the same gears.

The only thing to watch out for is there are two "series" of carriers (the ring gear mounting flange is a different thickness to match the ring gear thickness), but I think the break point is 2.73 so you should be OK.

73aussie455

Good news then, cheers.
Is it possible to tell without taking the cover off which of the two carriers I have?
If not, I guess its a good opportunity to freshen up the oil.

copper128

I'm not sure if this is a terminology issue, or if maybe you don't understand how this works. The "gears" (meaning the ring & pinion set, which determine the final drive ratio) are not "posi" or "non-posi". The ring & pinion set don't know or care if they're attached to a limited-slip or open differential. It's the differential which is different for "posi". You now have an open differential with 2.73:1 gears. To convert to posi, you need a limited-slip differential of some type (there are several kinds available for your axle type), but you could use the same gears you have now. If you want to change the gear ratio and add a posi differential, you'll need both a new differential and new ring & pinion set. Additionally, those parts have to match as to "series" - meaning the thichness of the ring gear has to match the position of the differential case ring gear flange. In the case of the corporate 8.5" 10-bolt, the cases are the same for 2.73 and numerically lower (called a "2-series"). There is a "3-series" differential for 3.08 and numerically higher gear ratios.
Make sure you know what you're buying before you order anything. It can be expensive and frustrating trying to mate the wrong parts together.

copper128

Also, just my opinion here, but if you're going to run ultra-low profile tires that are smaller in diameter than the original 15's, I'd recommend keeping the 2.73 gears and just install the posi differential. You should have no issues making clouds of smoke with a 455 and those tires

Fun71

Are you sure about this? I ask because I really don't know. I looked around online a couple days ago and found some conflicting info, then today I found this on Richmond Gear's web site:

http://www.jegs.com/p/Richmond-Gear/...44482/10002/-1
GM 8.5"
Fits 2.73 and Higher Carrier

If I'm reading this correctly, according to them 2.73 is a 3 series carrier.

BlackGold

Now wer're getting into terminology again.
3.08 is a higher ratio than 2.73, but it's commonly called a lower gear. So which is Richmond trying to say? I'll let someone else research it.

Fun71

I think Richmond makes it clear they are referring to "numerically higher" ratios as stated in the first block for the 7.5" rear:

GM 7.5"/7.625"
2 Series Fits 3.08 Lower Carrier, 3 Series Fits 3.23 Higher Carrier

monzaz

10 bolt 8.5 series breaks (gm gears)
2.41-2.56 2 series
2.73-5.57 3 series
1970-1987 28 spline axles
1988-2013 30 spline axles
10 bolt 7.5 and 7.625 (gm Gears)
2.14-3.08 2 series
3.23-4.56 3 series
1975-1988-89 26 spline
1990-2004 28 spline
12 bolt chevy (gm gears)
2.56-2.73 2 series
3.08 -3.73 3 series
3.90-6.14 4 series
1965-1972 30 spline
10 bolt 8.2 BOP 64-67 GM Gears
2.56-2.78 2 series
2.93-3.23 3 series
3.36-5.13 4 series
1964-1972 28 spline
10 bolt 8.2 BUICK 68-70 GM Gears
2 series 2.56 only
3 series 2.73-3.08
4 series 3.23 -3.42
5 series 3.64-3.90 and beyond
1968-1970 28 spline
12 bolt olds (28 or 31 spline bolt in axles 10 bolt ring gear) GM gears
2 series 2.56-2.78 28 spline only
3 series 3.08-3.23 28 spline only
4 series 3.42-5.00 28 or 31 spline
There are more rears but this is enough for now. Jim

73aussie455

Wow, thanks guys. Awesome info, once again great forum.
Monzaz = legend..

Only thing left for now would be how do I tell if its a 28 or 30 spline?
From the info I've read elsewhere and here I think I have a 30 spline.

1964f-85

Im running a protrax unit, love it!

monzaz

Now you have the axle spline range too. Jim

monzaz

Sorry - you have 28 spline axles.

Allan R

Thanks Jim, I was sure that the 3.42s needed the series 3 carrier, but another member here said I was wrong - that the GM 8.5 corporate rear series 2 would handle the 3.42's easily. I didn't feel like arguing and since I already bought and installed the limited slip you sent me, it was a moot point. As always, you're information is clear, easy to understand and to the point.

Here's maybe a dumb question. Can a 3 series open carrier be converted to a limited slip one by rebuilding the guts?

Fun71

No, the carriers are significantly different and the side gears and clutches can't fit into the open carrier. You could install an auto locker such as a Lock-Rite, but that would not be a LSD.

edit: I found some pics of an "S" spring style limited slip carrier (left) and an open carrier (right) to show the differences in them:

...........

nsnarsk65cutlass

Good info monzaz, i noticed you noted GM gears as the aftermarket has ring and pinion sets that work on different carriers.Nick

73aussie455

Good clear concise info, thanks monzaz.
I've copied the info to my Old's data file for future reference.

All good now, I'll go see how much I'm going to get ripped off here for an Eaton Positrack with 3:23 gears

monzaz, are you in the differential business?

76olds

Hey 73aussie455, I remember you mentioned to me that your up here in Canada along with me, Just an FYI.... I purchased all my rear parts from JIM (monzaz) and he shipped them to my door here in Orangeville Ont. I would highly recommend him. Awesome price with unbeatable customer service. Jus sayin'
Cheers Eric !!!

monzaz

yES IN SOME REARS like the 7.5 have thick gear sets that are traditionally 3 series but there are so many 2 series posi units that they decided to provide the customer with a built in ring gear spacer so to speak.
Or in the case of the 12 bolt OLDS A-body rear the 3.42 and 3.90 gear made by Richmond/ Motive gear are on the 3 series carrier (3.08 3.23) NOT the 4 series carrier they would NORMALLY be bolted to. This is because the 3 series carrier was just more popular and more were built from the factory.

Jim

monzaz

You are correct - You need the 3 series carrier for the 3.42 8.5 10 bolt corp rear. YOU can use the 2 series BUT with a ring spacer kit. YOU ALWAYS need the correct length (longer bolts when using a ring gear spacer)
Jim
J D

73aussie455

So 3 years on from my first post, I have purchased my posi center, 3.23 gears and rebuild kit. That should wake her up a bit.

I had an opportunity to bring it back from the US in my check-in luggage, avoiding the crazy shipping charges to down under.
Now, to have a crack at it myself or take it to a shop. I've never done a diff before.

Has anyone found a reference for which speedo gear to switch to? TH400 box.

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20170512_2121141_zpsvygrvzu1.jpg

20170512_2119381_zpsb8q02lh9.jpg

Olds64

Excellent!

oddball

There are a bunch of calculators out there. TCI's is nice because they also show the gears available:

http://www.tciauto.com/tc/speedometer-gear-calculator/

You have to pop out your unit to see what drive and driven you have now.

Fun71

Wow! The baggage handlers must have been cursing you.