C2 Posi Unit

[Philip Whitaker Member# 2024]

I need help or advice on the removal of the springs in the posi unit on my 64. One of the cages that hold the clutch packs lost two of the legs, one of which found it's way between the pinion and the toe of the ring gear, chipping about a half inch chunk out of the toe of a tooth on the ring. I've located a new cage, but I can't find anyone in my community that has the Kent Moore tool J-21175-4 that's shown in the service manual, which is required to remove the springs that preload the posi unit. I can't find a tool that might substitute, and I've tried to fabricate a tool, but the mild steel that I'm using gives under the pressure of the spring.

I'm sure others have come across this problem, and I'm wondering if there is any other solution than sending the entire unit to a specialty shop. I've been to the local drive line and transmission shops and all of the GM dealers in my area and no one has had any experieance whith this Dana unit.

Any help in this matter would be greatly appreciated. Thanks, Phil

[Joe L.]

Re: C2 Posi Unit

Phillip------

If you still have the original Dana unit in your 64 and it's, obviously, failed, I don't think that I'd fool around with trying to rebuild it. SERVICE parts for these Dana units are more difficult and expensive to obtain and some may not be available, at all. Failure of the unit has already damaged you ring gear. If you replace the R&P as you should, would you want to risk the same damage to a new set? Plus, what are the condition of the spider gears and posi clutch pack?

You can purchase a new Eaton unit through Tom's Differential of Paramount, CA. This will be a direct bolt-in. They're not cheap but that's the way that I'd go.

[Philip Whitaker Member# 2024]

Re: C2 Posi Unit

Thanks for your quick response Joe. I was thinking the same thing, that the units in the 63-64 cars were unique to those years because the design is flawed and there probably have been many failures. I wasn't sure if I could use a later design unit. Yes, I do have a new manufacture R & P, and I find out only after receiving them that I must also buy a new flange (yoke) for the pinion as no one services the 63-64 design pinion. It's starting to look like the only thing that will be original when I'm done will be the carrier housing.

I ordered the pinion yoke (flange) GM part #3878972 that is listed for 65-70 models. Do you think this part will work OK for me?

Thanks, Phil

[Mike M.]

Re: C2 Posi Unit

Joe, as usual, is right on. Trash the Dana(had many of the Dana posi cases split during hard accel. Sounds terrible and is. Get the eaton and forget about hurting it(unless you have 12" slicks and 500 Hp, in which case the half shafts or spindles will grenade before the eaton. good luck, mike

[Philip Whitaker Member# 2024]

Re: C2 Posi Unit

Thanks for your response Mike. I probably will replace the unit, but I'd still like to disassemble the old case and see if it's possible to fix. I've already ordered new cages. My only real concern is letting go of the $$$$ for the new case. I called Tom's Differential and they have new Eatons ready to ship. I will make a decision soon if I can't find a way into the old case. My 64 is the 250 HP with a PG, so I'm pretty sure the diff didn't get horribly abused in it's 40 thousand miles. I'm sure the new Eaton is the way to go, but I'm feeling the stress on my budget. My only thoughts on keeping the old unit are that when the car is on the road again, it probably will take me the rest of my life to put another 40 thousand miles on it and I know that I'm not going to be abusive with it. I called Kent Moore and they no longer list the tool needed to compress the springs so that they may be removed from the posi case. If I can't find a way to disassemble the case I'll be forced to fork over the $$$$.

Thanks, Phil

[Joe L.]

Re: C2 Posi Unit

Phil------

You're right; I forgot about the spline difference for pre-65 pinions. As far as I know, no one services the Corvette coarse spline gears used for 63-64. And, yes, the GM #3878972 flange will work perfectly with the new pinion gear and gear set and it will work perfectly with your existing housing. The 3878972 is also very similar to your original flange. It's not identical, but I don't think that anyone will notice the difference. Also, I believe that it's stronger. I think that the original 3741822 flange was cast nodular iron whereas the 3878972 is forged steel. The same drive-shaft u-joints are used with both your original and the 3878972.

The Eaton units are a bolt-in replacement for your Dana. In fact, GM always recommended that the Dana units be unit-replaced by the Eaton style. GM doesn't sell the Eaton units anymore, but, fortunately, they are available through aftermarket Eaton distributors.

[Joe L.]

Re: C2 Posi Unit

Phil-----

I think that the Kent-Moore tool that you need is one that was specifically designed for use with the Dana units. It was discontinued by Kent Moore MANY years ago. I'll bet that a lot of these tools have gone into the dumpster at dealerships since GM dealers stopped servicing the Dana units more than 30 years ago.

I fully understand the cost issues here and, for your application, you could probably get by with the Dana if you can fix it. However, you better check the other internal parts before you consider spending any more time trying to rebuild this unit. If you need other parts, you may not be able to get them and, if you can, the price may be very high. Tom's can tell you what is and isn't available and what they cost if they are.

[Duke W.]

Re: C2 Posi Unit

I believe Auburn also offers a cone type LSD for C2/3 carriers. They're availalbe in standard and higher preload versions, whilch is nothing but different preload springs as far as I know. Tom would probably be better able to give you the tradeoffs, if any, and the relative pricing.

I have the RPO G80 Borg-Warner cone type LSD in the 7.5" axle on my Cosworth Vega. It still has well above the minimum preload after 75K miles, about 4K of which are race track hot laps. The CV doesn't have as much torque as a 327, but a 250HP/PG is probably not going to stress the LSD that hard.

A few years ago I talked to Auburn, and they are the spun-off B-W division that made the cone type LSD in my CV.

Duke

[Philip Whitaker Member# 2024]

Re: C2 Posi Unit

Joe-Mike-Duke,

Thanks for your input. I have a new Eaton unit on the way. I called all over the nation looking for anyone who worked on the old Dana units. I found builders who new what the old Dana was but all of them said they would not or could not work on them. I also heard pros and cons for the Auburn unit. I decided to go with the Eaton because it's more popular and been around longer.

Thanks again, Phil

[Craig S.]

Re: C2 Posi Unit

Good choice Phil, Charley Smith who does the classes in Bloomington indicated that the 63-64 is best removed and tossed. It had problems, is not worth trying to rebuild, and the springs are way stiff. Even the Eatons take a fairly healthy effort to install the springs...Charley has mastered the "Chin install approach"...and demo'ed it in our hands on class....well, being an engineer I build a tool from tool steel and bolts to serve as an installation aid. Works better for me!...Craig