1971 GMC C2500 rear end questions

[Baime]

A description of my truck:
- 1971, GMC, C2500, leaf springs, stock 10 bolt rear end with 4:11 gears, stock T350 3 speed transmission, 8 hole wheel patterns.

Based on some older posts on this topic, it sounds like there are several brands/manufacturers of stock rear ends on GMC C2500s. I have no idea which one I have, but it has a 10 bolt pattern with leaf springs.

What I'm trying to accomplish:
- I would like to change the rear end differential gears in my truck to improve highway driving. A fairly knowledgeable friend recommended that I change the rear end gearing to 3:42, or change the trany over to a T400R4.
- If I'm changing out the differential, I was hoping to change over to a posi-traction differential in the process.

My friend recommended a guy he knows who changes out differentials as a side business. What this guy said:
- My old GMC rear end is an "odd ball" rear end, and there are very few replacement differentials with different gearing, and there are no posi-traction differentials available for that rear end. He said I would have to change out the rear end with a newer version to make these changes, and cutting and welding would be required. The newer rear end would not just bolt in.

My questions:
1) Is a new 3:42 differential available for my existing stock rear end?
2) Is a 3:42 differential with posi-traction available for my existing stock rear end?
(I'm confirming if that guy knows what he is talking about, that these are unavailable. I thought I found one an an Eaton page, but I'm not sure.)
3) If I decide to change out the whole rear end assembly, is there a preferred/recommended replacement rear end to use for my truck? Will it require cutting and welding?


Thanks for you comments.

JP

[StingRay]

You likely have an Eaton HO 52. Not called a 10 bolt. 4.10 is a tall as it gets in a 52 or 72 version of that. It is possible to use some components from a modern 14 bolt full floater but it's a lot of work. You'd be better off swapping to a 14 bolt full floater or a Dana. In a 14 bolt it looks like 3.73 is about as tall as you will go but there are 73-87 rear ends that should almost be a bolt in.

[Baime]

Stingray,
Thanks for the comments. A few more questions:
- A 14 bolt full floater or a Dana? Are these aftermarket complete rear end assemblies? Do they make an aftermarket rear end that would bolt right in?
- If I go with a rebuilt 73-83 GM rear end. Are there multiple versions of these for a 3/4 ton trucks with leaf springs and 8 hole wheel patterns? If yes, is there a preferred version?

JP

[Mike C]

A semi-float 14 bolt from a later truck can have gears as low as 3.23 if memory serves. It's GVWR is the same as the Dana 60 at 5500# which is higher than the Eaton in your truck which is 5200#.

This swap is fairly easy depending on your skill set. You have to be able to weld on new perches and a panhard bar mount.

I have this rear in my Jimmy from an '86 Suburban which had factory 4.10 gears. We put another in my kids Jimmy project from an '84 Suburban with 3.73 gears.

[dozerbill72]

When you say it's a 10 bolt is the cover on the backside round or is it rounded on top and bottom with vertical straight up and down sides?

[Baime]

Dozer,
A photo of my existing rear end.

This is the stock rear end on a 1971 GMC, C2500 with leaf springs, 8 hole wheel pattern.

I'm really surprised I can't get a new positraction rear end with higher gearing for this.


JP

[Baime]

Originally Posted by Mike C

A semi-float 14 bolt from a later truck can have gears as low as 3.23 if memory serves. It's GVWR is the same as the Dana 60 at 5500# which is higher than the Eaton in your truck which is 5200#.

This swap is fairly easy depending on your skill set. You have to be able to weld on new perches and a panhard bar mount.

I have this rear in my Jimmy from an '86 Suburban which had factory 4.10 gears. We put another in my kids Jimmy project from an '84 Suburban with 3.73 gears.

Thanks Mike C.,
So it sounds like I need to replace the whole rear end assembly, and you guys are recommending a "semi-float 14 bolt" or a "Dana 60" .

This leads to more questions:
- Are there two versions of each of these, for coil springs and leaf springs?
- What vehicles and years will have one of these rear ends?
- Is there a way for me to identify these, so I know I'm buying the right part?

A comment: My cousin thinks he has a later 3/4 ton GM rear end sitting around in his pole barn. I'm going out there to check it out. Hopefully this weekend.

Thanks for your patience on this. JP

[dozerbill72]

That is a Dana 60. There is almost every part in one of those still available. I know there is a 3.55 and 3.08 ratios available. With the cost of a new gear set, installation and all that goes with it a junk yard rear end my be cheaper.

[steves1967]

My 71 C20 has a Dana 60 in it from the factory. the rearend has never been out of the truck.

[Baime]

Originally Posted by dozerbill72

That is a Dana 60. There is almost every part in one of those still available. I know there is a 3.55 and 3.08 ratios available. With the cost of a new gear set, installation and all that goes with it a junk yard rear end my be cheaper.

Dozer,
Wow, that is some exciting news for me. It is also a completely different answer than what I got from the guy my friend knows, who changes out differentials as a side job.

In his defense, he never physically looked at my truck. I sent that same photo to my friend, and I assumed he forwarded it to that guy. I'm checking on that now. Maybe he just assumed that a 1971 GMC 3/4 ton would have the Eaton HO 52 rear end.

So based on the fact it is a Dana 60, I should be able to change out the differential to a different gear, and even go to a positraction rear end if I want.

If I get a used rear end, I would most likely change out the differential anyway.

You said this would be expensive. Any idea what this should cost for a positraction rear end differential? Parts? Labor?

Is there a preferred differential manufacturer?

Thanks again for your comments. JP

[Baime]

Originally Posted by steves1967

My 71 C20 has a Dana 60 in it from the factory. the rearend has never been out of the truck.

Steve,
What gear ratio is your rear end?
What transmission do you have in your truck?

I have 4:11 gears with a 3 speed automatic. I don't have a tach, but the engine is running fairly high rpms at 70mph. My guess is over 3000rpm.

John H.

[tdangle]

I have the same setup 350/th350 and 4.11 dana 60. Yes at 70 it is over 3000 rpm, probably closer to 3300. I put a 4 speed OD automatic (2004r) in and could not be more happy. Still have all the bottom end and cruises at 2000 on the hwy.

[Mike C]

My 70 GMC big block truck had the 3.54 geared Dana 60 and I ran 265 75 16 tires and it was pretty good combo for the road.

Expect to pay $250 for a set of gears, another $150 for bearings, and $400 at least for labor to set up a diff. Figure anywhere from $500-$900 depending on what you want for a traction device.

Obviously, you can save money buying used if you can find the right combo in the time frame you need.

Most likely a later model semi-float 14 bolt will have a Gov-Lock limited slip installed. While not my first choice, I am still running it in both the swaps I’ve done. And the 9.5” version appears to be much more robust than the 8.5” “Gov-Bomb”. Depending on your abilities, the later axle can be done fairly inexpensively.

[Baime]

Originally Posted by tdangle

I have the same setup 350/th350 and 4.11 dana 60. Yes at 70 it is over 3000 rpm, probably closer to 3300. I put a 4 speed OD automatic (2004r) in and could not be more happy. Still have all the bottom end and cruises at 2000 on the hwy.

Tdangle,
Thanks for your comments.
Your setup sounds nice. Is the 2004r OD automatic transmission different from the T400R4 transmission? Is there an advantage to the 2004r over the T400R4?

Is that a difficult conversion from the T350 auto?
- Shorten the drive shaft.
- New starter and flywheel?
- Does the shifting linkage line up, or is there some type of conversion kit?

Take care,

JP

[Baime]

Originally Posted by Mike C

My 70 GMC big block truck had the 3.54 geared Dana 60 and I ran 265 75 16 tires and it was pretty good combo for the road.

Expect to pay $250 for a set of gears, another $150 for bearings, and $400 at least for labor to set up a diff. Figure anywhere from $500-$900 depending on what you want for a traction device.

Obviously, you can save money buying used if you can find the right combo in the time frame you need.

Most likely a later model semi-float 14 bolt will have a Gov-Lock limited slip installed. While not my first choice, I am still running it in both the swaps I’ve done. And the 9.5” version appears to be much more robust than the 8.5” “Gov-Bomb”. Depending on your abilities, the later axle can be done fairly inexpensively.

Mike,
Thanks for explaining the costs involved. If I had a semi-float 14 bolt, would those costs be drastically lower?

My plans keep changing as I get more information from this group. I appreciate the shared knowledge. I'm very much a rookie regarding GM trucks, but I'm willing to learn.

I have some decisions to make. Based on all of these comments, I believe my best options are:
1) Leave the Dana 60 with 4:11 gears alone, and change over to a four speed transmission with overdrive. (T700R4, or 2004R?)
2) Update my existing Dana 60 rear end with the 3:54 gears (preferable with positraction), and stay with the T350 three speed transmission.

A comment: If the Dana 60 is a decent rear end with available parts, it doesn't seem to make sense to change out the whole rear end to a semi-float 14 bolt.

Take care,
JP

[Bigdav160]

The Dana 60 is a excellent bullet proof axle. I would keep it.

The benefit of a 200-4R conversion is that it is the same length as your TH350. Other than rigging throttle valve cable/linkage and wiring up the lockup clutch it's a bolt in.

The 700R4 is based off your current TH350 but is longer. It will require the mount to be pushed back and the driveshaft shortened. It has the same TV cable/lockup requirements as the 200-4R

[Baime]

I talked to my friend. He did say that he never sent the photo of my rear end over to the guy who rebuilds differentials. They talked on the phone, and I suspect that they both assumed that an older GMC 3/4 ton pickup would have some odd ball older rear end.

He still isn't very confident that I would have a Dana 60 as a stock rear end in a 71 GMC 3/4 ton pickup. I told him to look at images on Google. They seem to match up. I think the Hagerty website has an article about these rear ends, and they list 70s era GMCs as one of the applications.

So I'm having him confirm that it is a Dana 60, and if it is, to give me pricing to change the differential gearing and possibly adding positraction. I want to know if that is an option, and how much it would cost.

I'm still leaning toward changing out the transmission, and leaving the Dana 60 with 4:11 gears alone.

My friend agreed that the 200-4R is a physically easier replacement for my T350, but he also thought the 700-R4 was a stronger transmission. It sounds like he would recommend that I go with a rebuilt 700-R4 if I'm going to change the transmission.

I'm going to look at an old GM 3/4 ton rear end in my cousin's barn tonight.
Sounds like an adventure, or at least a reason to have a beer, and look at old truck parts.

Thanks again for everyone's comments. I'm starting to know a little more about this. Hopefully it will help me make a good decision. (Or a little knowledge is dangerous, take your pick.)


JP

[Mike C]

Most GMC trucks had the Dana 60 while most Chevrolets had the HO5200. But they can be found either way depending on what combination of factory options, what factory it might have been put together at, or what some "junkyard engineer" has swapped in!

The full-float setup on the Dana 60 8 lug is more heavy duty than the semi-float setup of the AAM 9.5" rear. But they are both factory rated to 5500#. Unless you load your truck way over rated GVW you are good either way.

If you can find the right gear ratio (3.23/3.42/3.73) with a Gov-Lok already installed, expect to pay about $200-$250. Less probably at a pick-ur-part. Brackets set you back about $50-$75. Then installation which will depend on how much cleaning, grinding, fitting, and welding you can do yourself.

Same is true of a Dana 60 though. If you can find a 3.54 setup with a TracLok then you are good to go. Some had PowerLocks as well and that would be the ideal setup. Most are open diff IME but that's limited to only a few trucks and they have all been 1970 GMC's. The 4.56 geared PowerLock equipped Longhorn I bought for parts had an open 3.73 gear set in it, so you never know what a PO did relative to the SPID.

[dozerbill72]

The overdrive transmission gives you the best of both worlds. You get the low end of the 4.10s with some top end on the hi way. Cost can be high again look to pay around $1000 for a good 700R. I did the overdrive route in my 71 c20 and love it. It was setup identical to yours when I got it, and screamed at hi way speeds. I used a 400 turbo with a Gearvendor because I had it. A rear end swap is still going to be the cheapest. More and more trucks are coming with some sort of traction device in them, which should help in your search for the "perfect" rear end.

[PepperTreeGarage]

Dana 60. They are also referred to as Dana 60 10 bolt. All parts are available, but not cheap. This one is in my '72 C20. I'm rebuilding it with 3.73 gears and a posi. Most likely with parts from Motive Gear. With a 4L80e in front of it.

[Slowguy]

Will you be doing any sort of thread detailing the procedure on this? I have an lq4 with a 4l80e and what you are doing to your rear end sounds like exactly what I’ll need!

[PepperTreeGarage]

Originally Posted by Slowguy

Will you be doing any sort of thread detailing the procedure on this? I have an lq4 with a 4l80e and what you are doing to your rear end sounds like exactly what I’ll need!

I wont be starting on the Dana 60 for a couple of months yet. But I can do a quick write up on the process.

[Ironangel]

Baime, thats a Dana 60 in your pic, KEEP IT! Dont let anyone who dont know what it is touch it! There is no such thing as a "T400R4"...Need to determine what gearing is in it now by pulling the cover and getting the numbers off the ring gear. You will also be able to determine what carrier you have at the same time. The Dana 60 only goes as high as 3:55 and as low as 7:17 with the carrier break at 4:10/4:56. Nows the time to ask yourself what exactly are your plans for your 3/4 ton Jimmy? Are you pulling heavy trailers and hauling heavy loads? Do you have a $3K to invest in a gear set, a posi or locker, and a questionable 700r4? Not trying to rain on your parade but transforming a 71 3/4 ton GMC into gas friendly daily driver is an expensive endeavor. The least expensive way out is a new 3:55 gear set and taller 16" tires and wheels...My daily driver has 3:73 gears turning 31" 275/70R-16's with a built 350 Turbo to pull a tandem trailer with another 4000lb truck on it. If mileage is paramount, then you cant afford to drive these old trucks..."Just Sayin"

[StingRay]

I agree keep the 60 if you can. Hard to beat a Dana if you are looking for tough and versatility. Based on the guy telling you parts were unobtainable I figured for sure you had an HO52. For the Dana 60 his opinion couldn't have been further from the truth.

[Baime]

Originally Posted by StingRay

I agree keep the 60 if you can. Hard to beat a Dana if you are looking for tough and versatility. Based on the guy telling you parts were unobtainable I figured for sure you had an HO52. For the Dana 60 his opinion couldn't have been further from the truth.

Stingray,
I agree. I found out that they didn't even look at the photo. They both assumed that I had the HO52. They are used to C20 trucks, not the GMC 2500s.