Open 2.56 to posi/limited slip 3.42 - ClassicOldsmobile.com

Open 2.56 to posi/limited slip 3.42

Old February 9th, 2019, 02:22 AM
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Open 2.56 to posi/limited slip 3.42

Hello everyone, im looking for some advices about changing gears and posi swap.

What i have is a 1971 Cutlass S with a 10 Bolt (cover) rear end and open 2.56 gears. (i decoded the stamping on the axle tube). Seems that this is the correct stock rear end, because i learned in this forum that the 350 2bbl cars had the very long 2.56 gears. My car was originally a 2bbl car, but i already mounted headers and swapped to the right Quadrajet of year 71.

So, which Posi unit/carrier yould you recommend for that rear end? Which gears? Will 3.42 be fine? Or too short?

are the some "everything you need for this swap - Kits" available?

hope that someone of you have done the same swap!

greetings!
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Old February 9th, 2019, 05:52 AM
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I don’t know the details on a corporate 10 bolt but you might want to do some facial exercises so you don’t get stretch marks from smiling when you get driving after the swap.
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Old February 9th, 2019, 06:35 AM
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Your corporate axle is just an 8.5" Chevy ten bolt. Parts for this axle pretty much grow on trees. Auburn, Eaton, Richmond Gear are all reputable brand names.
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Old February 9th, 2019, 06:38 AM
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I did this swap in my 72 Cutlass. Same principle and idea you have. I also changed my carb and intake to the proper 4bbl setup. Your carb should be a 7042150.

I ordered my gears and carrier from Monzaz (Jim Mitschke) who is an expert on rear end applications. His shop is called J.D. Race & Associates and he gives good advice and products. I'd suggest you PM him and ask. He sold me a GM Series 3 carrier and pressed the bearings on before it left his shop. 1972 350/350 never had the option for 3.42 gears but I chose them anyway. I had them installed at a local shop and I'm not happy with their work because the pinion seal is leaking. I've had it back to them 2x and still leaking. I'll likely do it myself in the spring.. When you change out your gears you'll also have to change out your transmission drive and driven gears to make the speedo and odometer register properly.

I looked up your axle because I was surprised that it's standard option was the 2.56 The 72 standard axle for Cutlass S is 2.73.

Last edited by Allan R; February 11th, 2019 at 03:19 PM. Reason: correct error
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Old February 9th, 2019, 06:40 AM
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Originally Posted by joe_padavano View Post
Your corporate axle is just an 8.5" Chevy ten bolt.
It is?? I was under the impression that Chevy rear ends were clips for rear axle, not bolt in.
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Old February 9th, 2019, 10:48 AM
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Originally Posted by Allan R View Post
It is?? I was under the impression that Chevy rear ends were clips for rear axle, not bolt in.
That is correct, but the carrier and gears are the same, so when you look for parts you can look up a Chevy 8.5 inch 10 bolt.

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Old February 9th, 2019, 10:54 AM
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Originally Posted by Allan R View Post
It is?? I was under the impression that Chevy rear ends were clips for rear axle, not bolt in.
The only difference is the outboard ends of the housing and the matching axle bearings. The carrier, gears, carrier and pinion bearings, seals, and cover gasket are all the same.
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Old February 9th, 2019, 05:30 PM
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I just had an Auburn posi unit put in my 71 442 with the 8.5 corporate rear end and it works beautifully, and came highly recommended by several local shops. No one in my area deals with Eaton, which I understand are the best. These shops sell installation kits which contain the bearings, crush sleeve and even gear paint to check the pattern.
After having 2:56 gears, you may find 3:42s to steep for relaxed highway driving unless you go to a 200R4 four speed overdrive transmission. Just a thought. 3:08s may be your best compromise.
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Old February 11th, 2019, 04:33 AM
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Originally Posted by Fun71 View Post
That is correct, but the carrier and gears are the same, so when you look for parts you can look up a Chevy 8.5 inch 10 bolt.
ok, great! I red that the 2.56 geared axles have a smaller carrier, so for example, its not possible to mount a 3.42 gear set on the original open carrier? Is that right?

i mean, if i buy a a 3.42 gear set and the right posi carrier unit it should be no Problem?


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Old February 11th, 2019, 08:18 AM
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Originally Posted by 71OldscutlassS View Post
ok, great! I red that the 2.56 geared axles have a smaller carrier, so for example, its not possible to mount a 3.42 gear set on the original open carrier? Is that right?

i mean, if i buy a a 3.42 gear set and the right posi carrier unit it should be no Problem?
You need the right carrier for the gear set. A 2.56 carrier will not be the same as the 3.42 you're planning on using. Why on earth would you even think of 3.42 on an open differential? Yes, if you buy the 3.42 gears and right posi carrier you will have no troubles bolting them up. Just make sure your pre-load is set up right.
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Old February 11th, 2019, 01:15 PM
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Originally Posted by Allan R View Post
You need the right carrier for the gear set. A 2.56 carrier will not be the same as the 3.42 you're planning on using. Why on earth would you even think of 3.42 on an open differential? Yes, if you buy the 3.42 gears and right posi carrier you will have no troubles bolting them up. Just make sure your pre-load is set up right.
no way, for sure i don't want to have a 3.42 on an open differential! This was just a thought for example/to understand it!

i wanna have a 3.42/posi combination.

Which posi carrier and brand for the gear set would you recommend? Dana for the gears?

i've never done a swap like this before. I watched a few videos and red some instructions. Looks like its kinda tricky to set the pinion depht right without this special measuring tool. Will it be a good idea to do this in the garage at home?
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Old February 11th, 2019, 01:25 PM
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The 2.56 ring gear is thicker than the 3.42 ring gear, so the ring gear mounting flange on the 2.56 carrier (2 series) is thinner than the flange on a 3 series carrier. This keeps the centerline of the gear teeth in the same spot for each setup.

You can either get a new 3 series carrier or use a ring gear spacer with the 2 series carrier. These are readily available from Summit, Jeg's, differential parts suppliers, even Amazon.

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Old February 11th, 2019, 02:11 PM
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Originally Posted by Fun71 View Post
The 2.56 ring gear is thicker than the 3.42 ring gear, so the ring gear mounting flange on the 2.56 carrier (2 series) is thinner than the flange on a 3 series carrier. This keeps the centerline of the gear teeth in the same spot for each setup.

You can either get a new 3 series carrier or use a ring gear spacer with the 2 series carrier. These are readily available from Summit, Jeg's, differential parts suppliers, even Amazon.

I want to change from these looong 2.56 gears to 3.42 and posi to get a bit better acceleration from off the line. Im sick of that feeling like the car is towed to a tree. But thank you for the explaination, now i understand the difference between series 2 and 3 carriers!
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Old February 11th, 2019, 05:04 PM
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I did the swap from 2:56 open to 3:42 posi on my 75. It made a huge difference. It will feel like like your car is stuck in second gear at 60 mph. But off the line it goes now. I never run much more than 65 mph with the new gears as the revs were up. I upgraded the tranny to a beefed up 200R4 last year. Now you can cruise all day 70 mph in lock up no issues.

Jeff
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Old February 12th, 2019, 12:58 AM
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Originally Posted by Tuffnut View Post
I did the swap from 2:56 open to 3:42 posi on my 75. It made a huge difference. It will feel like like your car is stuck in second gear at 60 mph. But off the line it goes now. I never run much more than 65 mph with the new gears as the revs were up. I upgraded the tranny to a beefed up 200R4 last year. Now you can cruise all day 70 mph in lock up no issues.

Jeff
ok, so do you think with the Th350, the 3.42 gear set is a bit too short?

I dont really wanna swap to a 200 - R4... and here in Austria we have 130 km/h (about 80 mph) on freeways. I wont drive much on freeways, but when i do, i dont wanna rev the hell out of my engine .. maybe 3.00 or about 3.20 is the better choice?

greetings
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Old February 12th, 2019, 08:21 AM
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3.42 gears are thirsty, and your engine will rev higher. Nothing it can't handle though. 1971/72 never offered the 3.42 with the 350 A body cars but did offer 3.23 and 3.08. FYI the only Olds A body cars that the 3.42 were were standard in was the W30 with 455. If you're concerned about having something that goes faster off the line and also gets decent performance on the highway, 3.08 is your gear. That said, in 1970 the Rallye 350 did have 3.42 gears....nothing wrong with them at all.
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Old February 12th, 2019, 08:41 AM
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Like Allan, said your engine will rev pretty good at 80 mph with 3:42 gears. I didn't enjoy the revs when cruising at 70 mph with 3:42 gears with my th350 tranny with 28" tires. Just me though.
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Old February 14th, 2019, 04:30 AM
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Thank you all for your tips and experiences!

I think i'll choose a 3.23 gear set. Sounds to be a good compromise.

greetings from Austria!
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Old February 14th, 2019, 09:28 AM
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I think the 72 Hurst Olds also had the 3.42s
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Old February 14th, 2019, 09:46 AM
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Originally Posted by Allan R View Post
3.42 gears are thirsty, and your engine will rev higher. Nothing it can't handle though. 1971/72 never offered the 3.42 with the 350 A body cars but did offer 3.23 and 3.08. FYI the only Olds A body cars that the 3.42 were were standard in was the W30 with 455. If you're concerned about having something that goes faster off the line and also gets decent performance on the highway, 3.08 is your gear. That said, in 1970 the Rallye 350 did have 3.42 gears....nothing wrong with them at all.
Actually, for the 1971 model year, RPO G92 3.42:1 gears were factory available with the L74 350 motor with any manual trans. With automatic 3.23s were the steepest gears offered with the 350. Of course, both MT and AT had the same 1:1 top gear that year, though the MT wouldn't have any converter slippage that would increase RPMs at speed. FYI, the difference between 3.42 and 3.23 gears is only a 5% reduction in RPMs.



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Old February 15th, 2019, 06:55 PM
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I have GM 3:42 & 3:23 gears in good condition.Also have correct BOP 8.5 posi units which are not the same as the Chevy one,as they don't have the recess for the "C" clips.Also it is my opinion that auburn posi units are junk.They are not rebuildable and send metal thru your new rear,the whole time it is in there because of their design.A male cone gets jammed into a female portion of the case,metal to metal (no clutches},When spent stops acting as a posi.Auburn came about after the Borg Warner cone type posi unit patent limitations ran out.I have Fact GM literature that states; if a spent Borg Warner posi unit is removed replace it with a Eaton unit A ring gear spacer should never be used,as it moves the ring gear off of the machined step of the differential that the ring gear semi press fits on to which centers and locates the ring gear properly,then the ring gear bolts bear all the load instead of just resisting sheer.
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Old February 15th, 2019, 11:15 PM
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Originally Posted by SmirchIs#1 View Post
Also it is my opinion that auburn posi units are junk.They are not rebuildable
I have no opinion on them being junk or not, but they are rebuildable by machining the ends of the cone and/or the wells the cones fit into. This restores the metal-to-metal contact you described. I learned this back in the 80s when a machine shop rebuilt my brother-in-law's cone style posi from his '68 GTO.
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Old February 17th, 2019, 05:55 PM
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Originally Posted by Fun71 View Post
I have no opinion on them being junk or not, but they are rebuildable by machining the ends of the cone and/or the wells the cones fit into. This restores the metal-to-metal contact you described. I learned this back in the 80s when a machine shop rebuilt my brother-in-law's cone style posi from his '68 GTO.
I hate to say but that's worse than a bandaid.By the time those units get to the point you are talking about(where you would machine them) the sufaces on the male and female parts of the cones are so rough they are literally throwing metal debri.Is that what you want in your rear?Also if you were to perform the machining or grinding operation on a "C" clip unit,you have just created a dangerous situation where you with lose a "C" clip on a corner,then corner the other direction and your axle comes right out of the rear.NICE HUH Was the name of the machine shop Morons R Us.If you want I can send you examples of spent parts free of charge.
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