The Stovebolt Site Forums The Shop Area Engine Shop 3/4 ton AD & C20 rear end, how different?

Seems like every time we get into the discussion of a 3/4 ton pumpkin swap there are always a few guys that don’t seem to realize that they are very different from the 1/2 ton. So I just thought I might post a couple of pics of a 1950, 3604.
http://rides.webshots.com/photo/2252244780098611668NhwSuM
I was looking at a 1969 C20, V8 3speed the other day and they sure do look the same. My housing of course uses leaf springs and the ’69 has coils but the center section looks just like mine. Did I read somewhere that the later, that is, post ’72 carriers had spiders with a fine spline and the earlier ones had a course spline? Is that the only difference and does anyone know if the spiders would interchange?
Denny Graham
Sandwich, IL

The 4 spiders don't have the spline, it's only that central gear thingy that the axle goes through. The older eaton is 17 spline (straight cut I believe) the newer 14 bolt gears has 35 spline (involute?).

The post '72 diff is a completely different design altogether. It's an integrated carrier design like the 10 an 12 bolts, only much larger. The carrier from that '69 should work just fine, and if there's a concern over the splines, snag the axle shafts from it too.

That diff looks just like my Eaton locker. Whether it's a locker or not is all on the inside. But it's an Eaton which I believe was made from '62 to '72 or there abouts,
Scott

John, the side gears certainly are splined on center section and I’m sure they are on any rear end that will interchange with it.

Well Scott, that thar picture in my first post is of a completely stock original 1950, 3604.

Now the rear end in this picture looks exactly like the ‘69 that I was talking;
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=014404

And that was what my question was about, will that pumpkin fit the 3/4 ton AD, ya know, it taint no 67 to 72 Suburban like every one is always talking about!

I gather from what your sayin Bill, is that it will interchange with the 4.56 that is in mine, and that the axles are the same except maybe for the spline. Is that correct?
Denny Graham
Sandwich, IL

Are you sure it's stock? I thought the 3/4 tons had a half torque tube rear diff through the AD series. I'm not for sure though,
Scott

on my 49 chevy 3/4 ton the "half of a torque tube" was the front half (hooked onto the trans.)

just so you know

Brian

See Scott! What I tell ya! Brian has it right, the 3/4 ton has a center support bearing and open drive shaft halfway back.
And yes, I'm sure it's absolutely stock.
Denny Graham
Sandwich, IL

Yes, and to be honest, I don't think that the spline will be different either. Alot of what comes up with the spline confusion is with the 1/2 ton 12 bolt. There was a spline change in mid-'65 from 17 to 30 on that diff.

Sorry Denny, I don't deal too much in those little trucks. Just helping you come up with an answers. I know mine's an Eaton under my 63 K20. It was supposedly only produced between '62 and '72. When I went through mine it had stamped numbers under the rear cover and under the "hogs head" into the housing.
Is the track width the same on the AD series compared to the later ones?
Scott

I don't know about track width, but the Eaton HO56 and HO72 was used from '47-'72. I think the one with the Detroit locker might be the one that's the '62-'72, but the basic axle is the same. There are some differences in the brakes, namely that the 1 ton had larger brakes, and the later (at least the '67-'72s anyway) have the drums mounted inboard of the wheel mounting flange instead of slide on like the earlier trucks.

That's what's under mine. It has the locker and I had to press the lug studs out to replace the drums. The drums were so bad, they actually broke when I started pressing.
Thanks for the clarification LB55. I have a spare Eaton under a chassis but it would be a real pain to try and get the ratio. It's rather loaded down right now. Maybe I'll try and get it over the week-end,
Scott

Thanks for the opinion on the splines Bill, trying to dig the info from the parts books is next to impossible, it just muddys up the water that much more. Man this is confusing, so your saying that the 3/4 ton rear end in mine is an Eaton, something????? And would all the Eaton, “something” center sections interchange??? That seems to indicate to me that there would be a fairly large selection of center sections available for the 3/4 ton AD. All this time I’ve been lead to believe that the ONLY center section that will interchange with mine and fit my housing without modification was the 67-72 3/4 ton Suburban, and I believe that the indications were that it had to be an automatic also.
Well this is more fuel to add to the fire that’s raging round my rear end! And I taint talking about Mexican food here!
I wish we could get a whole bunch of first hand experience in one thread that could put this “fire” out, once and for all. All of the junkyard I go to round here just give me a blank stare when I start asking these questions.

boy I just can't picture what you guys are talking about, "drums mounted inboard of the wheel mounting flange", how in the world do you do that???
Denny Graham
Sandwich, IL

Okay... According to info I have found to update my 49 3/4T: GM used Eaton HO52 and HO72 rear axles between 1947 and 1972. Most had a 4.57 reatio.

The 67-72 3/4T truck and Burb (either with A/T) had a 4.10 ratio. Some actually had a 3.90 ratio, but are equivalent to hen's teeth. The pumpkins swap for ALL those years. Randy's Ring and Pinion sometimes carry a 3.90 R&P, but they will only fit in carriers from (IIRC) 1962-1972.

So Options are, for older 3/4T: Keep the 4.57. Find a 67-72 3/4T w/ auto and swap the pumpkin. Find a 62-72 3/4T pumpkin and swap in either a 4.10 or 3.90 set, and swap pumpkins.

Re Brakes: Prior to 1951, Huck-style brakes are used (not-self adjusting). The drums slip over the wheel studs and are held in place by 2 flat-head screws (only to keep them from falling off when the wheel is removed).
Later models use Bendix-style brakes. These have the hub assembled to the drum, so removing the drum requires removing the hub and all that entails (yes, I prefer the Huck-method)

Simple answer: The 69 pumpkin will fit real nice, but if it was a factory 3-speed, probably has a 4.57 ratio, same as yours.

Good luck

According to the manual, the '55-'59 (bendix brakes) use the slide on style drums, too, same as the Huck. Probably a change in the early '60s. I've got a '60 manual, not handy at the moment, but I'll check it to see what it says.
Also, according to the info I have, there was no factory 3.90, however, there is a listing for a 3.54 for '72. I have actually seen 1, but i beleive that they're extremely rare.
Also, the HO72 generally had 5.13 gears, usually found only in 1 tons.

I thought Chevy and GMC used different axles? And what’s a IIRC?
I wrote to Randy’s Ring and Pinion and never received any sort of response one way or the other.
First option is out, I am gonna keep the 4.57, but on the shelf, I’m not going to run it.
Simple answer: Let me see if I got this straight; if I don’t take you literally, that is “get a ‘69 pumpkin” you are saying that in the worst case, all I have to do is find any 3/4 ton ‘62-‘72 center section and try to get a ring and pinion from Randy’s for it. Does that sound right?????
Denny G

For the most part, GMCs from about the mid '50s - early '70s used Spicer/Dana rear diffs.
What you're really shooting for is a '68-'72 center section, preferrably from an autmatic truck. Here's a list of the axle codes;
'68 w/4.11 gear; OT, NA, NB, WI, OW, HW, JV; '69 4.11; HW, JU, NA, NB, OT, OW, OY, TF; '70 4.11; TAA, THU, THW, TJU, TLN, TMZ, TNA, TNB, TOT, TTF, TWI; '70 C30 4.11; TIE, TIH, TLJ, TLX, TLZ, TMR, TNZ, TRN; '71 C20 4.11; TAH, TAJ, TJA, TJB, TJP, TJS, 2G, 2J; C30, TAK, TAL, TAP, TPZ;
'72 C20; RAH, RAJ, RJA, RJB, RKK, RKL, RKS, RKT, RJP, RJS, RKB, RKC; C30; RAK, RAL, RAP, RJK.
I also have the #s for the rare 3.54 rear, RKH, RKJ, RKP, RKR, RKD, RKG.

Denny,
IIRC = "If I recall correctly".

I have a spare Eaton under a chassis that's getting scrapped. It's the later version. I'll try to get the ratio if it ever stops raining. So what exactly are you looking for and what will you settle for? I think mine is a 4:11 but won't swear on it until I double check.
Scott

Forgot to mention, Jim Carter's web site has a tech article about this, probably better written than I can do. Check it out.

And yes, the taller ratio ring-and-pinion sets only fit in the newer carriers, but any carrier from 47-72 will fit in your axle (as long as it's from a 3/4T Eaton axle).

All the Eaton axles mentioned have a rear cover AND a carrier that bolts on to the front of the axle, sort of like a F<>d 9-inch. There is also a bolt on the left side of the pinion case which is NOT a fill plug. It is a tensioner for the pinion. Unless you arre adjusting it per the shop manual, don't touch it. Hope this helps... or confuses... Good luck

Thanks Bill, that really gives me some solid information when I go out to search the junk yards. Like I was saying, most of the time I just get a blank stare. I have to wonder just what the differences were when I look at all those codes. Were these the numbers for the axle assembly or the center section? And where am I going to find them, stamped? Tagged? There couldn’t possibly have been that many different gear ratios so I’m assuming that they have something to do with the brake set up also. One last thing, where in the world did you come up with all those codes??

OH, IIRC! I should know that one, I have enough problems in that area myself. I’m going to shoot ya an email Scott. No sense in talking a deal here.

Flyboy, thanks for getting this one off the ground, I think I picked up more information in this thread than in all of them I’ve read on the subject in the last year. Thanks again for participating and to all the rest of ya that jumped in the water.
I’ve probably read the JC tech article but didn’t pay it much mind. Up until I drove the 3604 for a few hundred miles I was adamant about keeping every thing absolutely original. This is gonna rub a few people the wrong way, but I’m beginning to weaken, there are three changes that I can think of for the better, the first is the gear. If it don’t show under the scrutiny of a judge then I’m willing to make the change. I think I’ve said this before; I doubt there are many VCCA’ers out there that crawl under and check the numbers on the center section at a show. The 4.57 will be on the shelf if the next guy wants to put it back in when my heirs sell it in 35 years. The other two changes are tubeless tires and insert bearings for the rods. The tires can be easily changed when going to a show.
I’m familiar with the ring gear thrust adjustment, it was one of the things that I checked and reset when I was rebuilding the under side of the old gal. No confusion, you have contributed more to clearing this up than anyone else so far.
I’ve got just a couple of other questions while we are on the subject. With a change to a later center section, do you have to alter the drive shaft length in any way?? And, along the same vein, will I be able to re-use the original axles, in other words will they engage the spiders properly, for instance could they be to long/to short? I believe we have already established the fact that the splines are the same. I had my axles out when I rebuilt the hubs and they looked like brand spankin new, absolutely no signs of wear on the splines.
The other thing has to do with the Eaton rear end. Two models have been mentioned here several times, the HO52 & HO72. Were they the only model Eaton axles used simply in various configurations? And once again, what would the difference be between the two??
Denny Graham
Sandwich, IL

Those codes should either be stamped on the axle tube or on the front face of the 3rd membber, depending on year. The coes I listed are for the 4.11 gear, there's plenty of other codes, too, for the 4.56 and 5.13 gears. I even came up with one for a 6.17! The info came from the Catalog of Chevy Truck ID Numbers from Cars & Parts.
Those 2 eaton axles were the ones used in the lighter trucks, the HO52 being the 3/4 ton version and the HO72 used in 1 tons. There really isn't much differences in them, other than perhaps brake size. In fact, I wouldn't be suprized at all to find a few HO72s under 3/4 tons, especially if they were equipped with the dual wheel option.

From what I remember from the JC site, the 3rd members are totally interchangeable in the year ranges we are talking about. The axle splines should fit as well. Seems to be one of the few totally bolt-in mods available from the junk-yard. And if your tires are big enough, you don't even have to jack up the truck to do the job!

Hey Denny,
I e-mailed but thought I would post too. Sorry to say the spare Eaton is a 4:56 non-locker. I'll start searching locally and see what I come up with,
Scott

After reading all the postings. I still don't know if I have a HO72 or a HO52, but needless to say it doesn't matter. I still either have a 4.57 or 4.11 gear. Either way it's bad. The best I can hope for is a 3.90 with a newer carrier. My question- Can a 14 bolt or Dana 60 rear be mounted under the 54 without it being too wide? I'm looking for a 3.73 or 3.50 gear that seems to be my best option.

You could possibly use a Dana 60 out of an '80s Dodge van, but it more that likely will not be a full floater. This will sacrifice overall strength, especially if you load the truck to capacity.

Look at a 14-bolt GM axle from a 80's or 90's Chevy or GMC. Some are real wide, but not all.

Again, use caution, many 14 bolts, especially in the lighter 3/4 tons, are semi float, and are not as strong as a full floater. With the GMs, it's easy to spot without even taking the hubcaps off, as the semi float 14 bolt has a much smaller center housing than it's full floating bretheren. the semi float uses a 9 1/2" ring gear, instead of the 10 1/2" gear in the full floater. And as suggested in my earlier post, look at vans, as they tend to be narrower than the pickups.

Thanks for the info. I'm not too concerned with the durability of the late model rearends. I'm not driving a dump truck, just the occasional trip to Menards or Lowes is probably all the action the truck will see.

And any Dana 60 is probably going to be far stronger than a stock rear end.

Just a tip, it is common practice in the hotrod/dragrace crowd to narrow a rear end. It is possible. Best to have a shop do that though. So if the rear end is too wide, get it narrowed.

If you aren't cpncerned about having a semi floater ( for the use you're doing, it'll be fine) then oe from the Dodge van will work fine. Most of them have the 3.55 gear.