1976Olds

I have a 76 cutlass with an open dif and want to switch to posi. What are my options.

Thanks

Fun71

Your car should have an 8.5" corporate 10 bolt rear, which has a lot of factory and aftermarket offerings for posi carriers. There are different carriers for different gear ratios, so you need to know what ratio the car has so that you can get the correct carrier. The break point for that rear is 2.56-down and 2.73-up.

1976Olds

I'm kind of new to this stuff, what is the break point.

joe_padavano

The gear ratio drives pinion diameter. Higher numerical ratios require smaller diameter pinion gears. This requires the ring gear teeth to move closer to the centerline of the pinion. Up to a point, you can make the ring gear thicker, but eventually this causes strength and tooth deflection issues. GM used different carriers to provide a selection of dimensions from the ring gear mounting surface to the center of the pinion. In the case of the 8.5" axle, all gear ratios from 2.73:1 on up use the same carrier. Only ratios numerically lower than 2.73 used a different carrier. This is referred to as the carrier "break point".

1976Olds

Thanks, so what you're saying is I need to figure out what my ratio is before I buy a posi carrier and gears

joe_padavano

Only if you plan to keep the current gears. If you are changing the gears when you install the posi, then just buy everything to match.

1976Olds

So I can keep all the gears I have if I buy the correct carrier. I'll probably go with a 3.43 or something like that but I have no clue what I have now.

Fun71

I would suspect your car came from the factory with a 2.56:1 or even 2.41:1 ratio, but that's just a guess based on what several friends cars had over the years.

1976Olds

Maybe, is there a code on the back that would tell me more. It came stock with a 350 4 barrel and I just swapped in a 425 and I wanted to put in a posi rear end. I've seen some s10 posies and ones out of trucks, would those fit. What vehicles interchange.

jensenracing77

Your best bet is to pull the cover and look at the number on the ring gear. This will tell you what you have with the most accuracy. It is not likely it has 3.42 gears but it is possible. Personally, I love 3.42 gears for all round street use.

rob1960

I changed my girlfriends 76 Salon over to posi a few years back. Her car, 350/4bbl came with the lazy 2.41 gears. I did the job on the cheap buying a used posi case off epay. Gm used the 8.5 rear well into the '80s( and maybe farther) in all kinds of vehicles, G bodies(rare) F bodies, B&C bodies. The one I bought came out of a full-size Buick Estate wagon. I also found her a nice used 3.08 ring & pinion. I just had to buy the new installation kit. I didn't want to go any steeper than the 3.08s since her car is mainly a cruiser but agree with Eric(jensenracing77) 3.42s are a nice all around street gear especially with your big-block.

JohnnyBs68S

I'm pretty sure all S10 rears are 7.5", same w/ 3rd and 4th gen F-bodies. So parts from those rears won't fit your 8.5" 10-bolt. And those 7.5" rears will break behind a BBO, so don't waste your time with them.

1976Olds

Ok I will do my best to find an 8.5 rear end. I was also wondering about the lockers that Eaton has for sale and if those would be good on a car that I daily drive or if those are strictly for the strip.

32redman

If you have the 2.41 or 2.56 gears and just want to make it posi. I have a carrier I'll sell ya for $75 plus shipping.

1976Olds

thanks for the offer, I'll check later in the week to see what I have and get back to you on that.

olds 307 and 403

Except for the last few years S-10 ZR2, all the rest are the weak 7.5" with the Gov Bomb posi. You have the common and decently strong GM 8.5" rear end. Full size trucks from 80's through 90 have the 8.5". The 90 and up trucks are 8.5" but have 30 spline axles vs your stock 28 spline.

brownbomber77

Something else to consider, you can use a 77-96 B Body (Caprice, Fleetwood, Roadmaster, Impala, Perisienne, Delta 88) rear axle that will bolt in. Most of the "bubble" cars came with a 3.08, 3.23, or 3.42 gear. Look for "G80" on the RPO tag.

I get it, a lot to absorb. I stuck a Roadmaster axle under my brother's '76 Cutlass and it had a 3.42 posi in it. Everything bolted up, including the brake lines. Installed new axle bearings and seals. Just needed to swap in a set of axles for the bolt pattern.

I Currently have an Impala rear axle with disc brakes under my '77 Cutlass. Had a Trutrac and 3.42 gears installed.

Need to use SVL 2022648 axles to keep your bolt pattern.

joe_padavano

Weren't some of the later ones the crappy GovLoc unit?

brownbomber77

Nah, they were the cone-type posi which, crappy no less when run with large wheels or generally abused. They won't blow up like the gov-locks do. Just won't have a posi differential anymore. This is just a less expensive alternative is all but I do understand going into it and just learning now it may be a lot to absorb. By the time you buy a posi and gears and pay someone to install it, usually in near $1,000.

1976Olds

So getting a new posi unit and gears will only run about 1k. That's what some people near me are asking for stock used stuff and half of them don't know the ratios or if it's posi

brownbomber77

Eesh. Well. Just doing it once, it will be worth the hassle. Just be sure it’s a reputable shop. I’ve had two builders say the factory GM gears are better than the aftermarket stuff. 8.5/8.6 stuff is abundant on eBay in decent condition.

you will have to change the speedometer gear in the tailhousing of the trans eventually. The Speedo will just read fast.

1976Olds

So I'm confused on what I'd need to buy. Would this work https://www.ebay.com/itm/GM-8-5-10-B...-/222606739852 another unrelated question is what are people using for tachs and where should I mount it

monzaz

Please be careful with these cheap kits they do sound good on the bottom line but they are knock off posi units - Gears are also a problem and will be minimally lapped causeing them to be noisey even when set up good. Also HEAT TREATING could be compromised on some of these gears and melt before they even get through break in.
Try to stay with a brand name posi and warrantied.
BE CAREFUL guys.... Jim

1976Olds

Thanks for the warning, could you please send me in the correct direction with a good company or dealer. I don't want to do this twice and am okay with having to save for a while in order to get a great unit that will handle some abuse. Also I was wondering about detroit lockers in something that I daily drive.

1976Olds

I've got a 74 buick century with a 4.10 posi I might be able to use. Would that be the same or is it different