Which rearend to use

[Branman1971]

I have a 10-bolt rearend in my 61 and i guess that it is stock. I also have a 12-bolt from my parts truck 63 and I don't know if it's stock either. But by these pics can anyone tell me if I have anything special with the 63? It has no drain plug either. Basically I'm just trying to figure out which rearend to use.

[Branman1971]

And the 63 12-bolt.

[AcampoDave]

It would be good to know the gear ratio for each particular rear end. Count the teeth on the ring gear and divide by the amount of teeth on the pinion gear. While your in there, examine the teeth carefully and note the amount of wear. Also look for chunks of metal hanging around in the oil. All things being equal wear wise, I'd use the one with the most favorable ratio. People say 12 bolts take more abuse, but I would still prefer gearing that complimnts my drivetrain and intended use of the vehicle.

[Branman1971]

Looks like the 10-bolt has 39 ring and 10 pinion so 3.90 ratio? Gears look really good inside so I will check the 63's 12-bolt tomorrow. I will most likely be running a bbc with th350 for starters, unless I find th400 or 700r4. Really not sure what is the best combo is for my 396. I'm not worried about going fast but I wanna cruise the highway at a lower RPM then what it did with the sbc 350 th350 and current 10-bolt axle.

[AcampoDave]

Yea, that 390 10 bolt is a "winder". Hope the 12 bolt comes up numerically lower.

[Branman1971]

Thanks Dave. Me too! The 12-bolt isn't a 12-bolt positrac is it by chance?

[AcampoDave]

Doubtfull, have a pal hold one wheel while you spin the other. If your pals wheel offers little resistance and stays put, I think it would be a "one legger"

[Captainfab]

You will have to move the trailing arm mounts but I would definitely go with the 12 bolt. There is basically no aftermarket support for the eaton HO32 aside from 3.38 gears. There is a lot of aftermarket support for the truck 12 bolt. Plus the 12 bolt was used up thru '82 in the C10's, so there are lots of used 12 bolts around if you need used parts.

If you go with a Th350 or 400, you can go with a 3.07 rear gear in a 12 bolt behind the big block. The big block will have the torque to get moving from a stop, and the 3.07 gear will keep your rpm's down on the highway. The 3.42 gear is a good all around gear, but depending on your tire size, the highway rpm's are still over 3000.

[Branman1971]

So I saw something interesting when I got under the 63 to check the gears. Apparently this truck had some sort of different set up at sometime. There is a round pocket fastened to the bottom of the frame and on top of the axle tube is where something was welded at some point. Is that where a bumper was? I took a couple of pictures.

[Branman1971]

Left side

[Branman1971]

Someone filled it with chocolate pudding!

[Captainfab]

It looks like someone at some time added a secondary oil spring to act as an overload spring. I have seen it on a few trucks.

That goo in the differential is exactly why I always recommend that when either changine differentials or a newely acquired truck, you should always change the gear oil. The differentials are one of the most neglected parts of a vehicle. With that differential, I would recommend changing the oil again in a few hundred miles....or less.

[Branman1971]

Point taken. Thanks. The 63 that I found the goo is a long bed. Sure that makes no difference. I'm gonna count the teeth tomorrow after they soak in my goo remover.

[Captainfab]

Just a FYI, the tooth count is stamped in the edge of the ring gear along with the part number and the date code.

[Branman1971]

No I didn't know that. What about pinion gear? Gotta count those huh?

[bama63]

Originally Posted by Branman1971

No I didn't know that. What about pinion gear? Gotta count those huh?

pinion count is on ring gear also.

[Branman1971]

Well hell...the 63 only has 3.72 gears. Guess that's better than 3.90 in the 61. I'm gonna use the 63 12-bolt since there are more gear options for it. Thanks for the info guys.

[Branman1971]

I'm finding gears online for the 10 bolt and some of them are 3.42 and such. Why would these not work with my ten bolt?

[Mackie]

A common mistake is confusing our 12 bolt truck rears with the more common 12 bolt auto units. The same is probably true of the 10 bolts.

[66Submarine]

Your rear end is the Eaton HO33, not what is called a ten bolt. The "ten bolt" is a totally different rear end made decades later.

[Branman1971]

Ah ok. That makes sense.

[Captainfab]

The only new gears available for the Eaton HO32 are 3.38 gears made by Yukon gear. They are $329.00 from Randy's Ring and Pinion.

http://www.ringpinion.com/b2c/Produc...rce=DiffWizard

Then you'll need bearings and seals, etc.

Like I said, I would just go with the 12 bolt. Even if you change the gears in it, at least you have a differential with better parts avaiability, more to choose from, and for many more years. Who knows how much longer parts will be available for the Eaton.

[Branman1971]

That's what I am gonna do. Thanks for all the info. So to change gears in the 12 bolt, is it just a ring and pinion swap or do I need to do bearings and seals and all too? I haven't taken one of these apart, but I can take ANYTHING apart.

[Clyde65]

If that is a 12 bolt with 3.73's would that not make it a 4 series which means you can not go lower on the gears? I believe that is correct.

[66Submarine]

Originally Posted by Clyde65

If that is a 12 bolt with 3.73's would that not make it a 4 series which means you can not go lower on the gears? I believe that is correct.

Yes, you'll need to come up with another carrier. Or you could get the whole assembly used out of a later truck that came with 3.07 gears.