damaged differential advice

[Duke W.]

Re: damaged differential advice

Fourth picture from the bottom... note that there is a section change at this point where the failure occurred. Most of the cross section has a relatively smooth surface, but somewhat less than a quarter of the area has a rough surface. This is a classic fatigue failure resulting from propagation of a crack.

The section change creates a stress concentration at the corner. Any small surface flaw in this area can result in a crack that will grow across the section. The slight relative movement of the two surfaces on either side of the crack from service stress "polishes" it. Then, when the remaining section is small, normal driving stress will cause the UTS to be exceeded resulting in a final failure and rough surface in the area of the final sudden failure.

This is the same thing that happened to my SWC at 30K miles and why I recommend that any used R & P should be Magnaflux inspected with particular attention to the section change. Any evidence of a crack at this point renders the R & P unsuitable for further service.

At the time my drive pinion failed in 1965 I was a ME sophomore at the U. of Washington and was taking the required materials science class. I took the broken drive pinion to class and with the help of the prof we analyzed the failure.

Duke

[Gary R.]

Re: damaged differential advice

Duke,
The pinion design on the 63-64 and some 65's was different then the later gears. The shafts did have a sharp corner and used those small "wedding band" sleeves on them. The pinion shaft was changed to a tapered one without the sharp corner and used the present longer sleeves. I'll have to check to see if I have a picture of the first design to show the guys.

I agree I don't like using used gears. The only time is when they come from a known car, buying used gears online or at a swap meet is a gamble in my opinion.

[Tim D.]

Re: damaged differential advice

Gary, thanks for posting the photos so the other members can see exactly what happened and how the rebuild looks compared to the original photos showing the damage.

Duke, thanks for your assessment of how and why the pinion shaft sheared in half.

Tim D.

[Duke W.]

Re: damaged differential advice

Duke,
The pinion design on the 63-64 and some 65's was different then the later gears. The shafts did have a sharp corner and used those small "wedding band" sleeves on them. The pinion shaft was changed to a tapered one without the sharp corner and used the present longer sleeves. I'll have to check to see if I have a picture of the first design to show the guys.

I agree I don't like using used gears. The only time is when they come from a known car, buying used gears online or at a swap meet is a gamble in my opinion.

Note: In order to avoid terminology confusion we should refer to the broken piece as the "drive pinion" to distinguish it from the "pinion gears" and "pinion shaft" that are located inside the differential case.

Clearly there was a design problem with the first design drive pinion - in particular the sudden section change, and it's my understanding that the first design was limited to '63-'64 models. Most are familiar with the later second design having a greater spline count for the companion flange, but I'm not sure of the rest of the details. I believe you are saying that the second design drive pinion has a tapered shaft rather than the step change in section of the first design. (A tapered design would be a clear improvement by eliminating the stress concentration at the section change.) Or did the tapered drive pinion show up later as a third design?

It would be great if you could show a photo of the first and second design drive pinions side by side, so we can see the differences.

The early design drive pinions were snapping in two back then, and they are apparently still doing the same, today.

Duke

[Gary R.]

Re: damaged differential advice

Here are new gears, in this case Tom's/US Gears 355's. You can clearly see the tapered shaft.



Here is a unmolested first design pinion, 336 ratio from an original CA 1965 open diff. The open cases were stronger then the Dana posi's but they both used this gear design. I don't know how long into '65 they were used. I may have the casting & stamp numbers on this '65 in my file. The sharp "corner" is right at the "wedding Band" sleeve exactly where Tim's sheared and probably the others as well. I'd bet most of the hard run 63-65 and up to 68's have been either blown up and scrapped or rebuilt by now. I see mostly untouched originals and never reuse those gears.

[Duke W.]

Re: damaged differential advice

The tapered drive pinion is clearly superior due to elimination of the stress concentration at the section change on the earlier design.

But it's not clear from you post when you think is tapered design went into production.

Early '65? Not until '68?

What's your estimate.

Duke

[Gary R.]

Re: damaged differential advice

Sorry about being not clear about the R&P and the posi cases. The 1st design R&P I found into '65 like the one shown. I've blueprinted a lot of the 65's and that was I think the only one I found the early gears in. Take a look again at it. Here's what I found with that set when I went back to take a look, dated 4/65, it was a 30 spline pinion & yoke where the 63-64 used the 17's.

The odd ball thing with Tim's 64 is the fact the bearing caps are the later type as well.I don't have an answer for that one other then what ever parts were left over were used and mixed in with the later ones?

The posi cases from 63-64 were Dana, then they moved up to Eaton(1st design from 65-68) that is what I meant about up to 68. The 65-68 Eaton weren't much better. Many times the posi cases let go first. The Dana's were weak and the 1 design very prone to cracking and then breaking. The castings were the problem. That changed in 69- 79 with the tear drop posi cases.

Of course by the mid 60's BB were coming about and the 10 bolts were still not capable of a lot of drag use or street abuse. Pepe Estrada and Tom Watt really started the building of these in the early 70's in Paramount CA. The herd has thinned since then, you can count on a hitch hikers hand how many still build them like that.


I went back in my files to another 64 original diff, here is the new & original side by side. Man, Duke you have me clearing dust off things now!!

[Duke W.]

Re: damaged differential advice

So is the following synopsis correct?

1st design ring and pinion - 17 spline drive pinion, stepped shaft design, 1963 to early 1965.

2nd design ring and pinion - 30 spline drive pinion, tapered shaft design.

In other words, the increase in spline count and change from stepped to tapered shaft design occured at the same time.

Never mind about the cases and carrier cap bolts. I'm just trying to understand the evolution of the ring and pinion design changes.

Duke

[Gary R.]

Re: damaged differential advice

Duke
Yes from what I've seen. It appears the transition was first to go to the 30 spline then sometime after that 4/65 gear set date, the tapered shaft came about.

I guess it was just part of the evolution of the C2, find the weak areas and correct them.
With the C2-C3 diff's they never reached the top level. When they got the R&P setup better, the attention turned to the posi case, that was corrected in 69, then a regression in my opinion in '71 with the snowflake clutches replacing the solid steels. The spiders got better in '71 with the 10-17's, then around '73 they started using soft face side yokes thru '79, the '77-79's were pretty sloppy as well once the shoulder bolt &washer RG bolts were replaced with flanged head bolts that were not loctite set. Many of these had the RG bolts back out and lock up. I remember seeing a few near new 78-79 diff being replaced because of that. Finally ending the run with the Dana Aluminum 80-82 diffs. These were the worst of the lot.

[Tim D.]

Re: damaged differential advice

The differential rebuild by Gary Ramadei was a SUCCESS! I am very happy to report I have my car back together and have had it out on the road the last 3 beautiful fall days. The posi functions flawlessly and is as quiet as a mouse with absolutely no issues, no noise, no leaks, no excuses. The new 373 gears are fantastically fun. My car is very quick around town and on the highway in 4th gear, I think I was running about 2700 on the tach at 60 MPH. I may need to double check that today when I go for a cruise.

I am very pleased with Gary's rebuild and service before, during and after the sale. He has called me several times to check on my progress as far as getting the car back together and getting it on the road etc. He has been invaluable with his advice and patience when I called him with installation questions etc...since I've only removed and installed a Corvette rear end/suspension etc twice in my life (once with the body off and once with the body on).

I will follow Gary's advice for break in. I am going to drive my car for about 20-30 minutes and then let it cool for at least an hour. I will do this for 10 times and then change the oil and posi-additive and then be set for life!!!!!

A HUGE thank you for everyone's help and advice on this issue. I am feeling good about putting another issue to rest and I am closing in on getting the bugs out of this restoration and being FINISHED!

Tim

[Duke W.]

Re: damaged differential advice

The fluid change after breakin is a very good idea, but Positraction clutches wear over time and the Positraction additive degrades with use, so for best long term service I recommend a fluid change about every 30K miles, but depending on the age of the owner and annual mileage accumulation, the time may never come.

Duke

[Tim D.]

Re: damaged differential advice

Duke,

As always, thank you for your input. I always value your opinion. I think perhaps I am supposed to change the oil and posi-additive after the first 500 miles rather than after ten 20-30 minute break-in / cool down sessions. I will double check with Gary and check my paperwork he gave me. Like you mentioned, as far as changing it in the future every 30,000 miles, I'm not sure if I'll ever put 30,000 miles total on this car. You never know.

Have a good night!

Tim

[Gary R.]

Re: damaged differential advice

Tim
Run the car through the heat cycles, change the oil at 500 miles. Add in the Lucas 85-140 gear oil and 1 bottle of GM posi additive. Good idea to change it at 30k or 4 years. Since you now have a drain plug it's a piece of cake.
Enjoy the car, the pictures look sharp!

[Duke W.]

Re: damaged differential advice

Why are you recommending 85W-140?

Duke

[Gary R.]

Re: damaged differential advice

Hi Duke,
I've been running 85-140 since I built my first custom diff. I use it my street cars in 20*-100*F weather without issue. You can use the Lucas 90wt as well.

Tom Watt always told me to run the 85-140 in both street and strip diff's and that was good enough for me. His diff's have held up pretty good for the past 35 years so who am I to argue the point. The Lucas oil clings better then the typical oil. I never use any of the other oils out there but everyone has an opinion on what is best.