Rear end help needed please...
 

I have a 68 12 Bolt diff running 3.08 gears and want to go up to 3.73. A guy has some 3.73 gears and "carrier" to suit for sale locally. The carrier is stamped 3866116. My rear is a 3 series and the carrier for sale I think is a 4 series. What I need to know is the 4 series carrier going to be compatible with my rear that has the 3 series components? Thanks for any assistance. Pete

Re: Rear end help needed please...
 

The carriers have a different off set that move the ring gear farther from, or closer to, the centerline of the pinion gear. That is because the pinion gear changes size. They call them series because a group of ring gears will fit a series 3 carrier and another group of ring gears will fit a series 4 carrier. Some one will correct me if I'm wrong but I think 3.42 is the cut off for the series 3 carrier. Anything above that and you need to go to a series 4 carrier.

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Series 4 = 3:73 and up, so if you can get the 3:73 ring and pinion gears with the carrier, it will fit in your 12 bolt housing.
the 3:73 axles are 30 spline

1G1YY, are the 3:73 gears out of a truck??, a 12 bolt car is different

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On another note, what kind of transmission does the truck have, and how tall are your rear tires? Revs will be 21% higher out on the highway, so things will be busy if you don't have super tall tires or a OD trans.

Re: Rear end help needed please...
 

The other responses have been correct, I would just add an comment to the last question. Yes, assuming that they are both "open" carriers and that the new one comes to you empty. The internals are the same, however, I would prefer to use the ones that were original to it. Also keep them together as a "set", once worn, mixing them is not so good.

There are likely guys who come on here and tell you how "they have done it and it's fine", but you are taking a risk. It might be, but then again might not. It's just "best practice" not to.
This is far more important with the actual ring and pinion, they get much more pressure/load applied to them.
The real issue in this swap is the used ring nd pinion themselves. That is a far bigger challenge. You would be far better off if you could actually see (still installed) or even take apart yourself, the old gears. The backlash and pinion depth are very important!
With brand new gears, there is some tolerance, they get to wear themselves into each other. Used gears have already done that. They need to go back exactly where they were, even if that doesn't meet the specs that sound ideal.

Re: Rear end help needed please...
 

Totally agree with Longhair

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Thanks to those that took an interested and responded.
I have now decided that used parts are too risky and will now look for a gear/lsd kit, esp since the gears I was considering are interstate 500 miles away..
Since I have a slightly modified and reasonably torquy 350TPI/T700 in my 62 Stepper with stock 29" wheels (currently fitted with 3.08 and open centre) and our open speed limit is 65mph I am thinking of 3.42 gears, though still considering 3.73s... esp as most of the driving will be in 50 and 60mph speed zones.
Google tells me there are many kits out there, but I would rather spend bux with a Forum vendor as long as the quality is reasonable and cost is fair,
There are limited choices here Downunder and the costs are substantial.
Any tips on this subject would be welcomed.
Thanks, Pete

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Good luck in whatever you decide and are able to purchase, how about a picture of the 62 Stepper, thanks

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I wouldn't have a problem with a used carrier and spider & side gears, and have done just that a few times over the years w/o marking their locations. However new gears and a shaft aren't too expensive.

While you're at it you might consider a limited slip carrier. My 69's 12-bolt has run an Auburn cone-type diff for around 15 years, and it still works well. The truck will lay down two equal length tracks of rubber, and has no nasty habits on wet turns. I kept the original ring & pinion but used new bearings, shims, and crush sleeve. Probably could have used the original pinion depth shim.

Also, if those 3.73 gears are real GM and in good shape, they may serve you better than new imported sets.

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I have run every gear in my cars from 2.78 all the way to 4.33's. The best all around gear for me was 3.42. It just depends how you are going to use it.

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The used gears are marked Excel..guessing they are Richmond.
Re pix of my truck which is a job in progress, but most hard work done.
I am not computer savvy and don't know how to put pix up.
Apart from that it has been a body off rebuild over 4years and nearing completion, 4 -5" lower, 73 disc brake front end, right hand drive and the list goes on.
Looking back I should have swapped the rear gears and fitted an LSD when the diff was out and on stands.
It wasn't until I took it for a quick drive that I realised that it will leave a single black line, both gassing in a straight line and when reversing up my inclined driveway,... and prematurely wear out the rh tyre!

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if you click on the POST REPLY button (lower left) GO ADVANCED button (at the bottom, right next to POST QUICK REPLY) you will go to a screen that has a paper clip along the top - click the paper clip and a page will pop up that allows you to browse for 1-5 pictures you can attach. As long as you know how to navigate to them, should become self explanatory from there. If not, please feel free to ask more questions.

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Attachment 2148676

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Thanks Jocko
Attachment 2148679

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Yes thanks Jocko, great pics of the truck, it's looking good.

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Nice looking truck 1G1YY!

This post interests me.... I suppose I didn't know about the 3.42 gears.

So I have open 3.08 (or is it 3.07?) in my 72 350/TH350 setup. I have toyed with the idea of the various options to go OD, which I know my current rear end won't be so good for OD. But I'm not 100% convinced I want to go OD (ROI on that cost). My tires are 275/60/15 on Rally Rims.

So it comes down to what I want to do and keeping in mind the "ifs" - if I do or if I don't do OD. Or do I leave things completely alone, as I keep telling myself. I even thought about just going positraction with my current gears but that seems silly to me - if going that far might as well do the gears too.

I have a feeling I wouldn't be happy with 3.73 on the highway if I never did OD but perhaps I'd be happy with 3.42 positraction either way (OD or not)? Heck everything around me in Texas is 65+ and if you are doing just the speed limited you are pissing people off LOL

Oh my question: Anyone running 3.42 positraction with a setup similar to mine? What is it like, street level, highway, MPG?

Re: Rear end help needed please...
 

I ran a 3.42 in a 55 Chevy car with 27.5" tall tires and Muncie trans with 1:1 fourth gear. Your tires are nominally 28" tall which is only around 2% taller.

Have to say the car was very busy on the highway at anything above 60-65mph. And that was without torque converter slip of 200-300 RPM or more, which you'll have to take into consideration. And I'd forget about 3.73 gears, unless the truck will seldom see sustained high speeds, or you plan to go with an OD trans.

You are right about OD not being a good fit for 3.08 gears. A buddy of mine has a 37 Chevy with a mild 350 and 700R4. And even with 3.36 gears, he normally keeps the trans in 3rd gear until around 50mph. And converter lockup is controlled with a switch that he doesn't engage until 60mph or more.

Looks like the factory did a lot of 700R4s with 3.42 gears, because it was all about fuel mileage, not drivability.

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Rich, You are correct. It is a 3.07 rear not 3.08...my bad.
And thanks...my truck is still a work in progress but getting close.
My tip is that you use an engine rpm calculator...it will be your friend.
I just did this with your setup....3.07 rear, 28" wheels at 65mph .
With a 3 speed auto your engine will be doing 2459rpm.
T700 4sp auto the engine is spinning at 1721rpm....much nicer, but at a cost in time, effort and bux to install a T700.
The economical way of achieving lower rpm's at highway speed would be to fit taller tyres.
I would not worry about a posi as that will not affect engine speed, just traction to both wheels esp at take off.
Just some food for thought...

Re: Rear end help needed please...
 

Don't forget torque converter slip.

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If you go with new gears look at the different post about brands of gears. It will steer you away from potential bad gear sets. There has been a lot of posts about noisy gear sets and people not being happy with them.