'63 Rear Diff Rebuild - Code CF 4.56

[Jason S.]

My car - a '63 SWC - currently has a 4.11 rear gear but it appears that the rear diff is original to the car and was a code "CF" 4.56. Two questions:

1) I would have assumed if a previous owner tired of the 4.56, he/she would have swapped down further (numerically) to something less than a 4.11. I'm told though, that the diff carrier for the 4.11 and 4.56 is different than those for a 3.70, etc. and to swap down into the 3's would have required not just a ring/pinion swap, but a diff carrier swap. The theory here is that a previous owner swapped down numerically as far as they could without doing a diff carrier change - and hence why a 4.11 is in my car now rather than anything else. Does anyone know how many castings existed in the midyear cars to accommodate all the axle ratio options?

2) It appears that Diff rebuild kits and rebuild services are widely available for '65-'79 cars - but not '63-'64s. Comments like "....cannot rebuild 1963-1964 Corvette differentials due to a lack of rebuild components for their positraction carriers...." seem common. Does anyone know what is NOT being reproduced for the '63-'64 Diff, and what the option for rebuilding is?

[Joe R.]

Re: '63 Rear Diff Rebuild - Code CF 4.56

You don't have a problem as the rear end casting can be used and the posi unit can be changed to and New Eaton unit of either 4 series or 3 series and you can use any gear combination you wish.

JR

[Gary R.]

Re: '63 Rear Diff Rebuild - Code CF 4.56

Hi Jason
First off Joe Ray is correct, you can use a new loaded Eaton or used Eaton posi in your housing. I have done many 63 & 64's and retained the original housing. BTW I always call it a housing but the correct term is carrier. The posi is called a case.


I would like to expand a little for you or others out there with a 63 to 79 differential. I have covered this before but it is good to go over it from time to time.

First the 63-64 diff's were new as was the car. As with any new design there will be some "bugs" to work out. One area was with the differentials. The 63-64's have some differences as compared to later units. They have the same bolt pattern so they will bolt into any 63-79 vette. The front brackets used were very thin and broke under some loading. The front hole in the housing was tapped for a 7/16-14 bolt, the rear hole was a thru hole. This changed in '65 to both holes being through holes. The upper 2 cover holes were partially exposed by '65 they were not visible. The rear covers used a machine thread fill plug to '65 then went to a 3/4 npt plug. There was a metal vent in the cover and that changed to a plastic one in '65.

Internally, the 63-64 used a wider bearing cap and grade early 5 bolts to retain them. The housings were narrower between the bearing pads from 63-66. The bearing caps changed in '65 and again in '71. The ring & pinion design used in 63- '65 was weaker then the mid '65 to 79 and aftermarket. The pinions had a square edge above the large bearing and a small crush sleeve I call a "wedding band" sleeve. The spline count is 17 on the pinion and a matching 17 spline 1310 pinion yoke was used. This design failed at that wedding band sleeve under heavy loads and sheared the pinion in 1/2 as anyone who attended one of my seminars may recall seeing on the display table. The pinion shaft design and spline count changed in mid '65 and the shearing problem was pretty much resolved. I have not seen one sheared yet. That meant the 63-E65 pinion yoke will not work with the later or new after market gear sets. All the bearings used were Hyatt or later New Departure Hyatt (NDH) anything else found in a 63-79 diff was added by someone other then GM at the time.

Now POSI was an option from 63-69 so there were and still are some open diff's out there, I still see them from time to time. They were in fact stronger then the 63-64 DANA posi offered. The DANA unit was a weaker casting with tabbed clutches, many times they broke and sometimes the case broke in 1/2. By '65 the switch was made to the better Eaton posi which I will refer to as the 1st design posi. This was an improvement but not a real resolution since they were very prone to cracking and used 10-18 spiders. By '69 the redesigned Eaton case was introduced with a better casting design. While those do crack, they don't crack as much as the 65-68's. Solid steel clutches were used in the '65 to '70 Eaton's.
In '71 the spiders changed to a stronger 10-17 design but the clutches were slotted to allow posi additive in get in between them in a terrible revision to stop posi chattering. It didn't work and only weakened the posi. When I take apart a 71-79 posi that was pushed hard I find the clutches broken into many pieces. So there was never a perfect vette diff built from the factory. Now many functioned fine, many times the abuse they were subjected to caused the failure. The ring gear bolts used were a hardened shoulder bolt with hardened lock washers. Then by '77 they were replaced with a flange head bolt, no lock washer or Loctite used and many backed out locking up the diff.

There were 2 series of posi cases used, 3 or 4 series. The difference is the thickness of the ring gear flange. A 3 series case can use a 273-308-336-355-370-373- 390 and now 411-433- or 456 gears. The 4 series case was used with the 411 or 456 ratios offered from the factory. Back in the 60-80's guys with 3 series gears converted to 4 series by using a spacer between the posi flange and Ring Gear, this is not recommended as the ring gear bolts would loosen over time and back out. The correct way is the way the factory set them up, using the correct case for the gear set selected.

So with all this said, if you have a original 63 diff it can be rebuilt as Joe mentioned with a 3 or 4 series Eaton, new gears, new pinion yoke, and new master kit. All you're going to save are the housing, caps, axles, and cover. Now the choice of a new loaded Eaton, with fiber clutches, large springs, and the spiders used would not be my choice. Building a common vendor type diff also is not the way I would go. The question on the new parts pertains to the posi parts, the DANA posi's. They are no longer available and again now worth rebuilding. What I see with a lot of kits is they are supplied with the correct shims for the 63-66 housings, they come with the thicker shims used from 67-79. They will not work on the 63-66 yet no one tells you that. Some kits use very cheap seals, other use caged axle bearings in place of the original full compliment axle bearings once made by The Torrington Co, here in CT.

Feel free to contact me if you have anymore questions.

Gary

[Gary R.]

Re: '63 Rear Diff Rebuild - Code CF 4.56

Here are some pictures I dug up of 63-64's.

Here is the DANA posi



Here is a DANA that broke in 1/2.



Here is a 63 with a 4 series Eaton posi in it. Notice the wider caps.




Here is the 17 spline pinion and yoke, now obsolete.



Here is a sheared 64 pinion

[Jason S.]

Re: '63 Rear Diff Rebuild - Code CF 4.56

Gary,
Thanks for the wealth of information. Forgive me, but there are still a few things I'm unclear on.

Let me first say this - my rear diff doesn't appear to have any issues functionally per se, except some leaks and overall rustiness. There is no significant gear whine, nor is it broken. I was assuming since the diff is coming out of the frame during chassis cleanup that it was worth having a specialist go through it. I wanted to get involved in a thread like this so I knew what to expect before opening up the can of worms.

With that said - perhaps I'm missing something - you have said that rebuilding my unit with an Eaton case is an option if it becomes necessary, but then conclude "the choice of a new loaded Eaton, with fiber clutches, large springs, and the spiders used would not be my choice". After that, you mention that the earlier Danas are "again now worth rebuilding" - even though you have mentioned they have several inherent design flaws, and many of the parts to properly rebuild them aren't available. Given that our cars (mine at least) will live a relatively pampered future, I don't know I will ever stress the Dana to the point of breakage - but the fact that parts aren't available would still be an obstacle to reuse the Dana. I'm still unclear on what direction you'd be recommending.

Would you have a specialist in the Detroit, MI area you'd recommend? It sounds like you have a shop that does this work as well?

Jason

[Gary R.]

Re: '63 Rear Diff Rebuild - Code CF 4.56

Hi Jason
Sorry for the confusion. I will expand some more on what I do and how it is different then the way the other rebuilders build these.

Given the weak areas of the 63-64 I would never assemble and use one, even in a mild application. I just would not want to chance it plus I can do the work so why wouldn't I? Now there are plenty who have done just as you questioned, basically install new seals, a gasket and oil and go. That might be ok for a diff that otherwise was functioning ok and one that will not see a lot road time. The picture of the sheared pinion came out of such a car, who knows what history it was abused to over 50 years and let go without any apparent abuse. Again as a rebuilder I would recommend or build one like that.

I would go with an Eaton posi case over the DANA any day of the week. The new loaded cases available from any Eaton dealer such as Summit or Jegs is about $550 and comes with the newest idea on eliminating posi chatter in a spring type setup. I would not use one of these because I don't care for the internal parts that come in them. I have rubbed the fiber material off with ease in my hands. The springs are huge to keep constant pressure on the clutches, which will certainly give you posi action, but in order to get them in the number of clutches are reduced per side. The cases are good but not as good as the NOS cases made for GM. Those are thicker at the large windows. Bottom line is you can use one as they are sold, this is what a shop will offer you, but I wouldn't. I build them to application and have NOS cases here and the formed 10-17 spiders. I use solid steel clutches and tune them without the springs. This whole process you can find on you tube just look up Tom's posi tuning. He is building a vette posi in that video and using the HP 30 spline axles in it. Finding a shop to build one like this may be a challenge due to time and cost or lack of experience building custom posi's. Then there are those that will tell you it will not work without the springs and plates, which is true if the tune is not correctly done. So that is how I do them and explain at the seminars I hold at various venues. I use the best parts to custom build a Eaton posi. The guys I have taught did the same and if you take your time may be able to do it. It is involved but so is pulling a body off the frame.

If you have the diff out or plan to then I recommend you mount it on a engine stand, pull the cover and see just what you have. Many 63-64's have already been worked on and you might find a Eaton or even an Auburn cone case in there.

If you want a stock rebuild you can contact DTS in Detroit, they should be able to help you.

[Gary R.]

Re: '63 Rear Diff Rebuild - Code CF 4.56

I forgot, you might want to read Jack Panzica threads from last Summer. His 66 vette had 300k miles on a 68 diff that finally locked up. You can see how he likes the tuned posi. It's a good read regardless.