The Stovebolt Site Forums The Shop Area Engine Shop Another Rear end question

What would make a good donor rear end for my 59 flatbed dually.
It is a 3/4 ton with a 235 , 4 speed with a propeller shaft brake. And the shop manual for a 55.2 says that with this set up it has a 1 ton rear-end.
I want to swap it out with a lighter dually rear-end.

What I want is a faster rear-end, newer brakes, smaller drums and a smaller pumpkin but I want to keep my 8 lug wheel pattern.
I would like even more to set it up for a newer 4X4 setup.

I'm not going to haul a house so I want a lighter faster setup for my 235 and a setup that will go with a 327 or a 350 if I decide to swap out the 235 in the future.

Thanks
Bill

Any 1 ton rear end is going to big, and I would not suggest going with smaller brakes. Try finding a 3rd member from a '67-'72 3/4 ton, they are intrchangeble with your current rear and I have seen them with 3.55 gears. This would be easier than swapping rears. If you do want to change the entire unit then get a 14 bolt full floating rear from a '73-'87 c/k 30. If you want to do a 4x4 swap get the Dana 60 from a k 30 as well.

Try a search in the archives on Eaton to swap the chunk to a 4.10.

Is there such a thing as a lighter duty dually rearend? I don't think so. A one ton rear is a one ton rear as far as duty goes.

You can swap it out with a dually full floating 14 bolt from 73 up and get some taller gears that way, it will be close in width. I have seen some dually's out of diesels with 3.42 or 3.08. Most of these have 4.56, 4.10 or 3.73 that I have seen.

You could swap it with a 9 1/2" semi floating 14 bolt and put dually adapters on it or run it with single wheels and that would be a lighter rear end with newer brakes, smaller drums and a smaller pumpkin with 8 lugs. These are easy to find with 3.42 gears.

I'v seen dually Chevies with the 9 1/2" semi float. All of them were '93 4x4 extended cabs.

I'm fairly certain that the only stock gearings in a '67-'72 3/4 Ton were 3.90:1, 4.10:1, and 4.55:1. It is possible that I am mistaken, but I am fairly sure that I am not.

I believe that the Half ton had a 3.55:1 ratio available, though. If you had the whole axle, you might be in business. IIRC, by that time they'd done away with the torque tube setup, so it may be as easy as bolting the thing in.

Just remember: Smaller Brakes equals Less Stopping power and that truck weighs a little bit more than I think you think it does.

The stopping power of your brakes won't change, but the amount of force needed to stop your truck will, as you accelerate. If you're doing 70, and you've changed your brakes to a smaller model, then you WILL have problems stopping when you really need to. And that really isn't a good time.

If you do reduce your brake hardware size, be sure to pack an extra pair of underpants behind the seat.

You'll need them sooner or later.

That's weird the 9 1/2 semi floater is only rated for 8600 lbs GVWR trucks. Remind me to never buy one cause a semi floating dually rear is dangerous...

A freind of mine has a '72 3/4 ton that has a 3.55 gear in it. It's a 350/turbo 400 standard 3/4 ton model, coil spring rear suspension. We checked the codes from the 3rd member against the Chevrolet truck number guide from Cars & Parts. On the 9 1/2' duallies, I've only seen 2 of them, they may have been some sort of aftermarket conversion off a light 3/4 ton, but they did say 3500 on the sides.

Huh.

Go figure.

You learn something new everyday.

I heard that.

Check the RPO Code, this is not an Eaton. I have seen this myself. It is a 72 with a Dana Rear End. I got all giddy when a buddy of mine called me about a 1972 big block automatic coil spring 3/4 ton he had found, he bought it for the big block and turbo 400. I knew it was gonna have a Detroit Locker and an Eaton 3.90 in it. It sadly has a Dana rear end. He grabbed the drivetrain and the rest is still in his yard.

Longbox55, if this an Eaton, please post the RPO code.

I've always wondered how to identify the rear. My cab has the warning sticker for the limited slip differential. It also leave 2 patches on my smooth concrete driveway when I spin em According to my shop manual, I have an Eaton 3:90 as it was the posi available that year for a 1/2 ton. How would I know for sure? It has a 10 bolt cover. That's all I can see. Are there numbers somewhere? This is a part of the truck I just haven't messed with yet.

What's a detroit locker?

If you are serious about making it four-wheel-drive you might want to start by looking for a front axle. The gear ratio front and rear has to be the same, so you don't want a rear you can't easily match to the front.

From the website:
Detroit Locker
The Detroit Locker maximizes traction by delivering 100 percent of the torque to both drive wheels. It is engineered to keep both wheels in a constant drive mode, and has the ability to automatically allow wheel speed differentiation when required. Enthusiasts and professionals use the Detroit Locker in many off-road applications. It is the only differential used in NASCAR

Often referred to as NO SLIP on the RPO code sticker when put in an Eaton Rear End.

That '72 definetly was an Eaton 3rd member rear. Personally I've only seen Dana units in Camper Specials with leaf springs. I also have seen 1 Camper Special with an eaton rear with coil springs at a local salvage yard. The GMC's used Dana axles on their trucks, and I have seen them with coil springs on the rear.

So, pardon my ignorance, but what is the difference between a "no-slip" and a "limited slip" differential?

My question is, what is wrong with the standard 3/4 rear end? I haven't finished my 53 3/4 yet, so i don't know what it will drive like. I'm looking to install a 350 V8 with auto trans. Will this setup rev to high at 100km/hr? If so can you get different gears to improve it.

Gents:
Just my $.02 worth.
A couple of years ago when I started on Iron Annie, my 52 3/4 ton pickup, I wanted to swap out the ring and pinion for a more interstate friendly rear axle [numerically lower]. Found out as is mentioned in ths thread that the pumpkin from to a 72 3/4 ton is the same. Contacted many sources, before finally finding out from the folks at Randy's that the highest [lowest numerically] ring and pinion ratio available for that rear is/was a 3.90:1. Randy's actually had made a special production run on them, because of demand. I believe they are now sold out. The existance of a 3.55:1 3/4 ton Eaton rear is a possibility as GM typically offered higher ratio rear axles in automatic equipped trucks. I've just never seen one in a 3/4 ton. I have heard that the 3.90:1 is the highest [again, lower numerically] ratio R&P that will fit the Eaton rear because the higher ratio R&P's are dimensionally larger and won't fit it the housing..Can anyone with hands-on experience with these R&P's verify that this is infact the case..I do know that when we put the 3.90 in the pumpkin on my truck that the clearances were tight,really tight. It would be interesting to dimensionally compare the 3.90:1 to a 3.55/56:1 to see what the size difference is.
I may just phone Randy's to see if they've got that information.

Looks like there is more to think about now.

Jimbo, the size of the ring gear is the same regardless of the number of teeth. So a 4.10 or 3.90 or nonexistant 3.55 eaton will be the same size ring gear

bigmacdaddy,
Check out the Detroit Locker Page f...evices than can be found in the archives

So, I wasn't mistaken... Too much.

Brendan:
It looks like you were correct.
The folks at Randy's said 3.90:1 was the highest [lower numerically]R&P ratio available on the 3/4 ton. My big block 72 /12 ton originally had a 3.55 [or 3.56.1] R&P which we swapped out to a 3:73 to give me a little more off the line but not enough to buzz it to death on the interstate.
Have a great holiday weekend everybody.

Hi there, me againn Looks like I stirred up a real hornets nest with this one. According to my resources(Chevrolet truck id guide) there was a 3.54 gear availible in '72 only. The # in the listng matches my freinds Eaton rear. Most of the one I have seen were 4.11 or 4.56 gears. It may have just been a rare option. The guide also does not list a factory 3.90 for any year 3/4 ton, only for 1/2 ton. I believe it only listed Eaton #'s, not Dana's. I could be mistaken on this as the book is not very clear on the 3/4 and 1 ton trucks, only the '59 1/2 ton with posi.

So does this mean I should be tickled about having the posi 3:90? And still, how do I know for sure?

The '60-'62 1/2 ton used an Eaton posi with 3.90 gears. I beleive the code on the axle should be MU. The '59 used the GMC Spicer 45 axle for the posi option, with a 3.92 gear. I have the manuals for both years and have done some research on thae posi Eaton 1/2 ton rear. It's a very rare part.