The Stovebolt Site Forums The Shop Area Driveline 3:73 for Eaton HO72 rear

Can anyone confirm that there were 3:73 available for the Eaton HO72 rear. I have been told they were used in 67-72 Eaton rears on Chevy trucks with BBC option.

No. Only options were 4.10, 4.56, and 5.14.

I have a 3.54, at least I think thats what it is. Its been a while since I installed it. Anyway, it came out of a late 60's with a big block automatic. But its a Spicer. You need to change the whole rear end which is no big deal. The above mentioned ratios are the only ones that will fit the HO banjo housing.

Thanks. I have a 4:56
Any suggestions on where to find a 4:10 third member?

They're about as scarce as a chicken with lips. Most of them got scavenged up by us round track racers in the mid-1980's. We ran 3/4 ton Chevy rear ends with 4.10's on 1/4 mile dirt tracks, with a Muncie 4 speed locked into 2nd. gear. The 1.88:1 2nd. gear gave us a 7.70:1 final drive ratio, perfect for a "long" 1/4 mile track measured around the infield. The "short" tracks were 1/4 mile measured around the outside, and we used the 4.56 gears at those tracks. Some guys tried running the Muncie transmission in 3rd. gear on the longer tracks with the 4.56, but then the transmission cases would split wide open from the torque load. Dirt track racing puts an unbelievable amount of load on driveline components.
Jerry

Ken,
I had never heard of the 3:73 for the one ton HO72.I have a 4:11 3rd member for that rearend. I'm thinking of posting for sale but no 3:73's. SAM

GMC's with the Dana rear end had a 3.73 gear available- - - -no such thing for the HO series corporate axles.
Jerry

Thanks for all the info...appreciate it

Denny, you're incorrect. The HO series Eaton rear ends were used clear up to 1972 in 3/4 and 1 ton applications at least, and they did come in 4.11's. There is a one year only 3.90 (1968, I think), which I believe Art Hauser used to make his limited run of reproduction 3.90 gear sets some years ago. I put 4.11's in my 55.1 Chevy NAPCO. One came from a 62 Chevy and the other from a 66 Chevy. Direct bolt in. You can see more information here on Rob English's 4.11 rear end swap from quite a few years ago. Since this article was written several other part numbers have been verified and a whole bunch of guys have done this swap.

http://www.oldgmctrucks.com/page0276.html

BlueChipper,
I ALSO YEARS AGO HAD BOUGHT A SET OF 3:90 GEARS FROM ART HAUSER FOR MY 56 CHEVY 3/4 TON NAPCO. AT THE TIME HE WAS ONLY OFFERING 3:90's AND 4:11's. I HAD HEARD HE HAD BEEN KILLED IN AN AUTO ACCIDENT. I'M SURE GLAD I HAD BOUGHT THEM AS SOON AS HE OFFERED THEM.
SAM

Denny,

I never said i knew where a set could be found. I found my sets about 10 years ago now. One came from a wrecking yard in Oklahoma, and the other came from a rear end shop that was liquidating parts. The 3.90 gear sets that were originally available in the late 60's were, as far as I know, extremely low production to begin with. I don't know how many gear sets Art Hauser shipped before he was killed, but I had heard the remaining stock was tied up in probate for a very long time and I don't know of anyone that managed to get a hold of them after that unfortunate event.

I know several people have gotten the 4.11 differentials over the past few years.

I would offer this one last thing... If there is anything I have learned it's that just because I've chased parts and not found them, doesn't mean they never existed. Surely you can't think that there is some conspiratorial gathering of old truck nuts that are out to spread the myth of the 4.11 HO52-72 differential? I wouldn't waste my time BS'ing you. If I recall, you are the guy that has the beautiful unmolested blue 54 3600. One of these differentials with the 4.11's would be a nice upgrade for that truck. Tom Myers in Miami Oklahoma would be a good source. I know he's had them in the past.

GM shows no listing for a 3.90 gear for the Eaton rear. The only one I know of was the early '55 sets that were made for the Napco 1/2 tons, which used the 3/4 ton left hand axles for the front, as the did not have a 1/2 ton version available. GM also has listed the later C20 axle as both a "4.10" and a "4.11", depending on what year specs you're looking at. However, it also goes further and clarifies that the 4.10 is the Eaton, and the 4.11 is a Spicer. That information is from the GM Heritage Site.

I just did a little digging in the forums where I had posted on this subject back when I was looking. It's been nearly ten years. Not quite.

I misspoke earlier. My first set, I think actually did come from Tom Myers, and at the time he said it was in a early 60's truck, I think 62, maybe 61... I can't remember for sure now. The post I found from myself stated that the second differential came from a 63 Chevy 1 ton, and it had been completely reconditioned by the rear end shop I bought it from. The PO was swapping for a Dana with a posi. My 4.11 has a Detroit Locker. I paid $650 for it and had about a 14 hour round trip drive. That was approx. 2006.

In any case, I digress, because the conventional wisdom at that time was that the early 60's trucks were not born with these gear sets. I can't confirm this personally because of what my experience was, but supposedly the 67-72 carrier is the one that accepts the 4.11 gear set. The telltales guys have stated is to look for big block automatic trans trucks with coil springs. They generally have the 4.11 gears. I don't have time to search for casting numbers but I know there are a few that have been documented. I'd go get the numbers off of mine for you but the truck is in storage several hours drive south of me.

Longbox55, I cannot confirm the 3.90 3/4 ton set from the original run. I only know what I've been told, and I know that Art Hauser reproduced them for a short time before he was killed. I found a couple posts that reference that set as originally being a 1970 only offering, which makes my earlier recollection of 68 void.

As far as 4.10 vs. 4.11, that's semantics at this point. The math comes out the same basically. That's like when someone tells me that my truck isn't a 4.57, but a 4.56, even though 4.57 is stamped right on the data plate. You'll find that all of the early documentation of trucks equipped with eaton rear ends, shop manuals etc. regarding half ton trucks will state that they had 4.11 rear ends up through 53. Eaton rear ends in half tons from 54 and up are referred to as being 3.90, and Spicer rear ends are referenced as being 3.92. That didn't stop them from putting a Spicer rear end with an Eaton front end on NAPCO trucks starting in 57 for GMC.

I know nothing about rear axles. However, I like to see documentation, if it is available.

PDF of 1962 K10 C10 P10 axles
JPG of 1962 K10 C10 P10 axles

The makes of the axles are not shown (I think).

I know Denny was looking for documentation (he and I like to hear/read anecdotes and recollections and opinions, but if you are looking for parts, documents are nice).

Is the information at the links above pertinent to this discussion? If not, I'll delete this post.

Specifications Manuals are available at the GM Heritage website. I'll look in other years, if that might be helpful.

Denny, yes, I was talking about an unrestored and unmolested 54 3604. I recall it being in incredibly good shape and to me, yes, it is beautiful as it is. I am confused though... You say now you have a "4.10" center section ready for your 54, and that your 50, which I'm going to assume is also an 8 lugger, already has one. Are you saying that you're not debating whether there is a higher ratio gear set available but whether it's technically referred to as a 4.10 or a 4.11?

Tim, I think those docs would be pertinent to 1/2 tons only. This is mainly about 3/4-1 tons. I am willing to concede that my differentials may not have been born in the trucks they came from. They may well have originated in something between 67-72, even though they were pulled from early 60's rigs.

I do not see 3/4 ton axle info in that Spec Manual.

In 1966, the HO 52 is shown as a 4.11 or 4.57 on page 13 of the axles section, and it appears that in 1969, at least according to the GM heritage docs, that the Eaton was referred to as a 4.10 in the 3/4 ton trucks. I would like to state for the record, that I was never debating the semantics of 4.10 vs. 4.11. I was weighing in because the information the OP needed was what his options for finding higher gears are.

https://www.gmheritagecenter.com/do...hevrolet-Trucks/1966-Chevrolet-Truck.pdf

https://www.gmheritagecenter.com/do...hevrolet-Trucks/1969-Chevrolet-Truck.pdf

Different ring/pinion gear tooth counts are involved:

37/9 tooth gearset = 4.1111111

41/10 tooth gears = 4.1000

I believe the Dana rear axles used a 41/10 gearset. The Eaton gears are 37/9.

Jerry

Jerry that makes perfect sense. My problem was with the quote from Denny, which has now been retracted, saying that there "never" was an Eaton HO 52 with a 4.11 gear set. I believe I have linked documentation above that clearly shows a cross section of an Eaton HO 52 (5200 lb axle) with available ratios of 4.11 and 4.57 in the 1966 info packet.

I have a eaton that I just changed out into my 48 GMC 1 ton 3 weeks ago that has 41/10 gears, sorry no pic

That is interesting. I don't know which configuration I have in my truck. It would appear that more than one ring/pinion combo was used by Eaton. I believe the 1966 packet stated 37/9.

One of the Chevy 4X4 setups used a 3.07 axle ratio in the front differential and a 3.08 in the rear- - - -different manufacturers for front and rear axles! I believe it was a full time 4X4 with a differential built into the transfer case.
Jerry

Thanks for the info. I assume with Art's passing... his source is gone...

Denny, my name is Josh.

If you follow this link, and go to page 83 of the .pdf, which is page 13 of the axles and suspension section, you will clearly see the cross section of a HO-52 or HO-72 center section. You will furthermore see the gear ratio of 4.11, and 37/9 tooth count, optional on C, P, and K 20 series trucks. This is dated August 1st of 1965, for 1966 trucks.

https://www.gmheritagecenter.com/do...hevrolet-Trucks/1966-Chevrolet-Truck.pdf

The same diagram can be see on on page 87 of this pdf, or page 20 of axles and suspensions for 1967

https://www.gmheritagecenter.com/do...hevrolet-Trucks/1967-Chevrolet-Truck.pdf

I won't speak further about the 3.90's other than I recall when Art was selling them and I was too poor to buy them, and I know at least one guy that did buy them, and I've known him for over 15 years. His name is Sam Hagopian and he's no liar.

You asked for documentation. There it is.

Apparently for whatever reason, Eaton changed configuration at some point from 37/9 to 41/10 and therefore changed the ratio.

They changed the format of the documents in 68, 69, 70 etc. My assumption is that the ratio of 4.10 refers to the new 41/10 HO 52

If you continue to argue this point,you're arguing with GM, not me.

Why get anal-retentive about a 0.01 turn difference in a gear ratio? It's obvious that both gearsets exist in Eaton third members. For whatever reason, GM used both tooth counts at various times.
Jerry

This is exactly my point, albeit much more succinctly expressed... They both existed. The original quote from Denny that "they never existed" to me was counter productive. It's semantics at best when you consider that the goal of the OP was just to get some higher gears for his rig. His quote could be enough to put someone off of finding a solution. I find it sad that someone with as much knowledge as he obviously has would not be more constructive and helpful.

BlueChipper,
Your right... the ring gear on the 4:11 is stamped 1966. And as of today i had put that 3rd member in for trying out...... and self satisfaction reasons. In an extra 1 ton dually 1956 3800 H-072 EATON rear axle I had laying around. And every thing so far seems to be fine so far. 3rd member dropped right in, axles slid right in. Have a few more things to check later this week.
THANK'S
SAM

Going back to Ken's original question about a 3.73...

Ken, looks like you are trying to pick up top end cruising speed. You have an answer to your 3.73 question. And finding a 3.90 might be a bit of a unicorn. Let's say a 3.73 was available...what would you pick up on top end at same RPM from your current 4.56? You would pick up 18.2%.

You would pick up 10% by switching to a 4.10.

How do you pick up the balance if you can't find higher gears? Increase your tire circumference. You can make very precise estimates if you know your current tire circumference versus a proposed circumference.

Here is cut and paste from something I posted a couple years ago that you might find helpful.

If you do either or both, and you want your speedo to be accurate, you will need to buy an easy-to-install adapter that can be custom built to your needs. I got mine from Jim Carter's.

I have a '57 3/4 ton with a 235, 4 speed SM420 and A/C. I think it is about max'd out with the 4.10 and the 8R19.5 tires. I find that I use the granny out of the hole when the A/C is running. The larger brakes on the 3/4 ton handle this just fine, but I would not be comfortable pushing top end any further.

The results listed below would not have any material difference if you find a 4.11 instead of a 4.10.

Hope this helps,

Matt

From an earlier post---

Several posts have inquired about actual performance increases when looking to increase top end. For those interested in my field test results...

Testing was done using a GPS and calibrating MPH on the GPS to the reading on my speedo.

Switching my '57 3600 from STA Super Transport 8x17.5 bias plies to Bridgestone 8R19.5 12 ply radials gave me a pick up of 10% MPH. (Good luck finding the 19.5" rims!) I anticipated an increase of 7.5% when I did the calculations based on published tire specs by both manufacturers (based on the difference in circumference), but actual increase was 10%.

Changing my 4.56 rear end to a 4.10 gave me another 10%, exactly as the math indicated that it would.

Independently, either of these alterations would pick up 10%. Doing both gives a compounding effect and actually picks up 21%.

Before the change, my comfortable highway cruising speed was 52 MPH. I can now run 63 at the same tachometer reading.

Matt, that's a fantastic post. That is exactly the kind of field testing needed for real world comparisons.

I will be running basically the same combination of gears and powerplant in my 55.1 NAPCO. I may run a 261 instead of the original 235 though since I have one here. I have 19.5's, which were on it from the time it was upfitted, and between the 4.10/4.11 (not sure which tooth count(s) I have) I'm hoping for a much better top end than I'd have gotten with the 4.57's and stocker 15's. I'm sure there is a way to figure out if I'm losing anything to drag with the extra gears in the transfer case and front end, but it should still be an improvement. Now, what remains to be seen is if the Brownie Junior overdrive I found will fit between the trans and the transfer case. Fingers crossed. I am fairly sure it's about a 20% over (or under depending on which direction it's mounted) unit.

One thing people seem to forget is that these engines are not fragile little wallflowers- - - -they were built to run. A quick look at HP and torque specifications in one of my Motor manuals shows 1954-62 235's max HP rated at speeds from 3800-4200 RPM depending on the year of manufacture. Max torque is around 2K RPM. To hear the opinion some of the guys with lace on their drawers, any speed above 2500-3000 is risking a catastrophic engine failure. Bullfeathers! Make sure the rod bearings are good, and plant that right foot on down!
Jerry

Great thread but I have a few questions 'cause I'm not always the sharpest knife in the drawer.

1-Are the HO52 and the HO72 rear ends identical as far as third member size, axles and bearings? Is the difference in designation just because of the models they were used in, 3/4 vrs. 1 ton?

2-Will the 4.11,(37/9)third members used in the C30 one ton trucks fit regardless of year?

3-Are all 4.11's (37/9) Eaton rear ends?

4-Are all 4.10's (41/10) Dana rear ends?

Thanks, Jim

Jim, I can give you a definite answer on #4. I have a 4.10 in my Eaton rear end.

On #1, I think that the pumpkin is interchangeable on an HO52 or an HO72 if that helps, but I do not know the answers to your other questions.

On #2, as with others on this forum, I did not know that Eaton used a 4.11 in this rear end until I was educated by this string of posts. As long as the pumpkin is from an Eaton HO52 or HO72 and is going into an HO52 or HO72, is should be a simple bolt in regardless of the gear ratio.

Matt

Jim,

HO52 and HO72 are 5200 lb capacity and 7200 lb capacity.

I believe that the difference is going to be primarily in bearings and brakes, but one could argue that a 4.10/11 gear set is decidedly less adequate in the grunt department than a 5.14 gear set. My assumption would be that is a factor in the capacity as well. Generally speaking though, HO52 is 3/4 ton and HO72 is 1 ton.

As has been illustrated in the previous posts, 4.10's and 4.11's were made for Eaton axles. The 3rd members will swap between the two. As far as I know the axle spline counts were always the same. There will be mechanical differences between the 3rd member assembly from year to year, but they should all bolt into the housings and mate up with the axles regardless. I can't speak for the Spicer axles. Just to be clear, these early axles are Spicer axles, and predate the purchase of Spicer by the Dana Corp. So if you are working on one of those, you'll see them referred to as Spicer in the books.

AFTER INSTALLING THE 4:11 3RD MEMBER IN THE 3800 1 TON DUALLY REAR-END. I HAD CHECKED EVERYTHING OUT. DIRECT BOLT UP "EATON" FOR H-050 AND H-072 AXLE. THE 10 BOLT 3RD MEMBER IS A ROUND FLANGED EATON NOT A SPICER. QUESTIONS CALL 262-895-6779
SAM

In reply to Blue Chipper, do the 5200 lb and 7200 pound capacities refer to each side of the carrier? I know where to look for bearing information on the 46-54 trucks but not on newer models from the 60's and 70's. I'd like to see if the bearings are different sizes.
Safe to assume my '48 with 8800lb GVW will work with either the HO52 or HO72?

The capacity of the axle is exclusive of the GVW. The GVW is the weight of the vehicle plus it's maximum payload, including passengers and fuel.

Your 48 with an 8800 lb GVW was at the top of the ratings for a 1 ton truck, that being said, an HO72 would still have been the axle it shipped with, and it would have been the same axle that shipped with a C30 in the 60's and 70's, with minor mechanical differences within the carrier assembly.

I can't tell you for certain what the capacities refer to but my guess is that you may drive yourself nuts if you over think it. Understand that all things being equal, if you put a 4.10/4.11 differential into an HO72, you are in effect derating that vehicle's carrying/pulling capacity. 8800 lbs would probably technically be incorrect after that swap. Also understand that the spring packs, as well as tires and wheels also factored into that GVW rating of 8800 lbs.
The bearings will have numbers on on them you can use to source new ones.

Does that answer your question?

Yes, that answers my questions, thanks! Obviously, I do like to drive myself crazy but that's how I learn.
It's funny how every answer leads to new questions and your post that listed the axle weight ratings is a good example. On my '48 with the 8800lb. GVW the front axle is rated at 3500 lbs., so I guess the rears are listed at 5300 lbs. If that is correct it would seem that the HO52, which you listed at a 5200 lb. rating would be close to the original but the HO72 with a 7200 lb. rating would be a better choice.
The key piece of info you provided was that the C30 pumpkins will work for me. That gives me a better chance of finding what I'd like.
Thanks again for your help and thanks for everyone's input, information and advice! Jim

I'm glad that helped, but I feel like I need to clarify something for you.

You can't add the front Gross Axle Weight Rating (GAWR) and the rear GAWR and arrive at a Gross Vehicle Weight Rating. (GVWR, or GVW) The two types of ratings are mutually exclusive.

Your 1-ton truck was equipped with a HO72 axle, not an HO52. The combination of gearset, bearings, and brake equipment made it an HO72, which has a rating of 7,200 lbs. on it's own. If you were to properly equip a chassis and vehicle to support that kind of payload (uncluding the weight of the vehicle and chassis) that is the maximum that axle would be capable (with a safety factor engineered in) of handling, which is not only carrying, but also stopping. The same goes for the front axle.

The GVWR is determined by looking at the capacities of ALL assemblies and adding a bit here and subtracting a bit there to arrive at a rating that both satisfies safety, and regulations.

The HO52, again rated at 5,200 lbs. is basically the same axle, but again the combination of gears, bearings, and brakes, reduce the rating of the axle by 2000 lbs.

Now, here's something to think about, and may put into perspective how it is that combined GAWR does not equate to GVWR.

My 1955 1st series 3600 is equipped with (2) HO52 axles, originally w/4.57 gears, currently with 4.11 (or 4.10, I'm not sure of the tooth counts). The GVWR on the door tag is 6900 lbs which is the highest rated 3/4 ton for that year. Now, this truck is an upfit, which predates factory 4x4's by a couple years in Chevrolet. If you were to combine the two axle ratings you'd come up with a 10,400 lb rating. That'd never happen. When the factory trucks started being released, the front driving axle option only added 600 lbs. to the gross vehicle weight and the combination of spring packs, wheels and tires could vary greatly from truck to truck, thereby changing the GVW in stock configuration greatly.

So, your truck has an HO72 in it's original form with 5.14's and 1 ton brakes. If you want a 4.11, you'll be pulling them from a 20 series truck, most likely, not a 30 series unless the diff has been swapped already.