Rear-End and gear questions

[Ole Greenie]

Hey Ya'll...Looking to get a few answers concerning rear gear components.
Situation: 1970 CST S/S with an 12 bolt open 3.08 rear gear set/diff out of a 75 C10. I am looking to change the gear set while installing a posi unit.

While looking around for gear sets and posi units, I have come across some confucing info so I am unsure what to get.

1) I plan to install the 3.73 set with a posi unit but if it is a 12 bolt, but does this mean the number of bolts on the rear diff cover or the number of bolts holding the ring gear in place?

2) Is the ring gear an 8.5" or an 8.6"? Are they interchangeable?

3) Is the pinion gear a 30 spline or 28 spline gear? Which one will I need?

Just seems like GM could have made it all simpler by making a large case and a small case housing and standardized internals except for posi and gear sets. Then you have to take into account a housing for either leaf or coil springs.

Any clarification would be much appreciated. Thanks guys.

[Fitz]

The term "12 bolt" refers to the number of bolts holding the ring gear to the carrier.

The ring gear for a GM 12 bolt truck rear is 8.875 in in diameter and is not interchangeable with other housings.

The pinion is 30 Spline, 1.438"diameter gear.

Gearsets for ratios from 2.73 through 3.42 use the small carrier. Gearsets for ratios 3.73 through 4.56 use the large carrier. They are not interchangeable.

Go here:

http://www.drivetrainspecialists.com/index.html

you can look up what you need, then buy it anywhere.

[Ole Greenie]

Originally Posted by Fitz

The term "12 bolt" refers to the number of bolts holding the ring gear to the carrier.

The ring gear for a GM 12 bolt truck rear is 8.875 in in diameter and is not interchangeable with other housings.

The pinion is 30 Spline, 1.438"diameter gear.

Gearsets for ratios from 2.73 through 3.42 use the small carrier. Gearsets for ratios 3.73 through 4.56 use the large carrier. They are not interchangeable.

Go here:

http://www.drivetrainspecialists.com/index.html

you can look up what you need, then buy it anywhere.

This is great info! So, since I am running a 3.08 open the best I can hope for is a 3.42...the most gear I can have for my carrier. I'am thinking that this would be a good choice for an LS6 454 mated to a 700R4, since I don't want to turn too many RPMs. What do you think? Much appreciated!

[gearys 5600]

12-bolt means- the rear cover has 12 bolts holding it to the housing, 10-bolt- 10 bolts holding it to the housing.

Most 12 bolts have a 3-series carrier, 3.08 ratio to 3.73, But you can run a 4.11 gear or bigger with a spacer ring on a 3-series carrier.

[ERASER5]

Originally Posted by gearys 5600

12-bolt means- the rear cover has 12 bolts holding it to the housing, 10-bolt- 10 bolts holding it to the housing.

Uh no. 12 and 10 are the ring bolts. The cover just happen to be the same number of bolts on a GM12 bolt and a GM 10 bolt. A Dana 60 has 10 bolts on the cover, but it is not a "10 bolt." A Sterling 10.5 has 10 bolt on the cover, but it is not a "10 bolt." Many Oldsmobile diffs had 12 bolt on the cover, but only 10 bolts on the ring gear, not a 12 bolt.

[Fitz]

Originally Posted by gearys 5600

12-bolt means- the rear cover has 12 bolts holding it to the housing, 10-bolt- 10 bolts holding it to the housing.

I can show you a GM housing with 12 bolts in the cover that uses a 10 bolt carrier. Sorta puts a big hole in your theory about bolt count...don't it?

[Ole Greenie]

Thanks for the answers guys. I sure would like the 3.73 gear set, but I've chosen the 3.42 because I've got the small case and the BB LS6 has plenty of torque at low RPM.

I will be running it on the HWY occasionally for a period od 160 miles each way. ..maybe twice a month. Not that the LS6 gets much better fuel mileage on the HWY but I think a taller gear will be easier for the motor.

Appreciate the help. This board Rocks!

[JJchamp48]

Originally Posted by gearys 5600

12-bolt means- the rear cover has 12 bolts holding it to the housing, 10-bolt- 10 bolts holding it to the housing.

Most 12 bolts have a 3-series carrier, 3.08 ratio to 3.73, But you can run a 4.11 gear or bigger with a spacer ring on a 3-series carrier.

So I don't suppose this works the other way, if you have a 4:11 carrier and want to put in a 3:42 gaer set?
Thanks,
Vickie

[Richard H.]

I'm tired tonight but I believe you can go to a 373 gear set but that is it. Check with auburn I'm sure that is right.
Look here.
http://www.drivetrain.com/parts_cata...d_pinions.html

[Shaky]

Originally Posted by JJchamp48

So I don't suppose this works the other way, if you have a 4:11 carrier and want to put in a 3:42 gaer set?
Thanks,
Vickie

I don't believe so since they'd have to thin down the gear. As a result it probably wouldn't have enough rigidity to hold the gear tooth tolerances during manufacturing of the gear.

[hayhauler71]

Originally Posted by Ole Greenie

This is great info! So, since I am running a 3.08 open the best I can hope for is a 3.42...the most gear I can have for my carrier. I'am thinking that this would be a good choice for an LS6 454 mated to a 700R4, since I don't want to turn too many RPMs. What do you think? Much appreciated!

Richmond gear makes 3.73 and 4.11 for the carrier you have you just need to read the small print in the adds. With a 700r4 trans i would highly recommend 3.73 gears. Hope this helps.

[SSedan64]

Originally Posted by Ole Greenie

This is great info! So, since I am running a 3.08 open the best I can hope for is a 3.42...the most gear I can have for my carrier. I'am thinking that this would be a good choice for an LS6 454 mated to a 700R4, since I don't want to turn too many RPMs. What do you think? Much appreciated!

The Carrier determines what gear ratios you can run, not the Rear End housing. If you're changing the Open Carrier to a Posi Carrier you can get a 3 or 4 series Carrier so you can run what ever gear you want.

[hugger6933]

pretty sure that the 12 bolts will have 12 on the cover and the carrier. The 8.5 in ring gear will be a 10 bolt.I also think that the spline count you are talking about one is 10 and the other is 12 bolt stuff. I am positive that if my recallection is off someone will correct me. Jim

[cdowns]

it might save you a bunch of money in the long run by taking it in and having a pro do the work

[Ole Greenie]

Any additional thoughts?

[Shaky]

Originally Posted by Ole Greenie

Any additional thoughts?

Setting up a rear yourself includes three basic things: pinion depth, backlash, and patterns.

Pinion depth is the trickiest. Your new gear set will have a number stamped on it. Your job is establish that depth. It is measured from the centerline of the carrier/ring gear and usually requires a special tool. The truck 12-bolt has a crush sleeve to set-up the pinion bearings that's fun to work with too (go a little too far and you have to get another one).

Once pinion depth is established, you bring in the backlash by shimming the carrier either left or right until the "gap" between the gear teeth equals the backlash number.

Then patterns get checked by painting the teeth with pattern grease and then spinning the gear set with a torque applied to the pinion. Then you get to interpret the results.

Maybe you knew this already, but if you didn't you should be aware before you start. I did one for my '66. It took me a long time - but I tend to agonize over this stuff.

Of course I went from 3.73 to 3.08 so we should have just swapped rears instead.

[Ole Greenie]

Originally Posted by Shaky

Setting up a rear yourself includes three basic things: pinion depth, backlash, and patterns.

Pinion depth is the trickiest. Your new gear set will have a number stamped on it. Your job is establish that depth. It is measured from the centerline of the carrier/ring gear and usually requires a special tool. The truck 12-bolt has a crush sleeve to set-up the pinion bearings that's fun to work with too (go a little too far and you have to get another one).

Once pinion depth is established, you bring in the backlash by shimming the carrier either left or right until the "gap" between the gear teeth equals the backlash number.

Then patterns get checked by painting the teeth with pattern grease and then spinning the gear set with a torque applied to the pinion. Then you get to interpret the results.

Maybe you knew this already, but if you didn't you should be aware before you start. I did one for my '66. It took me a long time - but I tend to agonize over this stuff.

Of course I went from 3.73 to 3.08 so we should have just swapped rears instead.

You know, You kind of wonder how these are mass produced at the factory with all this precision! I ompletely understand the precision requirements of all these adjustments. I would rather have a specialist/machine shop install the new gear set to make things easier for me...not to mention they will warranty their work too.

Sure would be nice to find a friend who would be willing to set it all up for an afternoon or two of college football and beer! Y'all take care!

[Shaky]

Originally Posted by Ole Greenie

You know, You kind of wonder how these are mass produced at the factory with all this precision! I ompletely understand the precision requirements of all these adjustments. I would rather have a specialist/machine shop install the new gear set to make things easier for me...not to mention they will warranty their work too.

Sure would be nice to find a friend who would be willing to set it all up for an afternoon or two of college football and beer! Y'all take care!

Well - the factory is making a huge volume of these, so they can justify some exotic, expensive tooling that allows then to do these things very precisely and very quickly.

Guys like you and me are only making one.

[N2TRUX]

What you don' mention is what size tire you plan to run. The height (and width) will also have an impact on what will be your optimum gear ratio.