Still speculating on what happened from last nights carnage and have yet to get the car back into the garage and up on stands to inspect the damage but it looks like I may have blown the rear. The car is a ‘68 L68 with a 4 speed and HD 3.08 posi.
Last night returning from a cruise I pulled away from a light a bit more aggressively but not flooring it. Didn’t intend to spin the tires but I did and as I was about to shift into second BAM BAM CRUNCH BAM ! There were sounds that came that are impossible to describe. It was obvious from the sound of it under me some gears had come apart and there was mechanical chaos. Forget trying to put it in gear to roll again as it sounded horrible. I
was lucky enough to roll into a clear parking area out away from traffic. Couldn’t ask for a better place for a flat bed to scoop you up.
Now with the the car stopped I catch a breath, the engine sounds fine and the clutch works though it has that nasty acrid burnt smell.
With the engine off I could roll it in neutral backwards but only about 3’ forward to hear mechanical interference.
When the flat bed came we put the e-brake on and slipped the bed under the car from the front and up to the rear wheels then with the brake off dragged the rear wheels about 1-1/2’ onto the bed. I noted that the right rear rolled and the left dragged. To unload it I sat in the car and rolled it off backwards without issue. Today the car is in my driveway outside my garage. I tried moving it but the left wheel doesn’t want to let the car roll forwards or back. I lifted the left rear wheel with a floor jack and I could rotate the wheel slightly to hear a crunching sound. I used a mechanics stethoscope to see where it was originating but it is about equal from the diff to the outer wheel bearing. Can’t see any obvious damage to the halfshaft u-joints (which are relatively new). Also remember this is a posi diff, so I’m not liking that I can rotate the left rear even slightly with the right rear wheel on the ground Not a good sign.
Harbor Freight has a sale on wheel dollys so I picked up a pair to enable me to roll the car into the garage. I’ll tackle that tomorrow.
Gonna be another sleepless night trying to work out in my head what I’m dealing with.

Just mounted these Coker Silvertown redlines and knowing what these cost you might believe me better when I say I wasn’t looking to spin these.


At least it still looks nice and I didn’t wreck it

 

Good luck Lt. I trashed one in a Suburban once and it sounded kinda like that.


ps. Take pictures!

 

That's a real bummer Mike.
Quick check, if you have a small telescoping magnet. Pull the fill plug, stick the magnet down in there and see what come out.

 

Jack up the car, both wheels off the ground.
If the LH wheel is binding, mark your camber position and remove/disconnect the 1/2 shaft. Now see if the rear wheel is binding or the inner axle of the differential.
You have a 68, they had weak posi cases, often they crack and implode. Possibly a ring gear bolt backed out, there really is only the outer axles and the differential. Once you pinpoint the area you can inspect.
If you need outer axles or diff work, don't swap for exchanges. Build them right, not cheap, or you will be doing it again.
Good luck, if you have direct questions, you can always contact me.

 

Ya I do and I was thinking of doing exactly that. Kinda afraid it’ll pull out parts.

 

I see that Zip does rebuilds on your posi, about $600 + parts. Gives me a baseline but I have a differential shop near me thats very good. If my inspection shows the rear is busted I remove it myself and bring it to the shop. Thinking the posi is destroyed. Hope I’m wrong.
btw thank you ,I may have to get back with you on this.

 

Questions to ask anyone selling or rebuilding a diff
If the posi is broken you’re going to have to check the gears and axles
1-what posi is used
2- is it new or used, if Eaton what revision
3- what type of spiders are used
4- what type of clutches are used
5- do you know what polished and tuned means referring to a posi
6- what type of RG bolts are used
There are more questions than these but if you get answers like “stock”, “textbook”, “I build race cars “, I had Corvettes when I was a kid “. Reconsider things If you’re told they don’t rebuild them and get them elsewhere you should look elsewhere.
I have been fixing junk from experts for years

 

Just to add something to think about. I went back to my files and pulled this example of what I see a lot- from some well-known places. This goes back years, but things have not changed for the better.

This diff was an exchange, rebuilt diff from a very well-known vendor. The customer bought it and put it in his garage for a year. He had me blueprint his steering box and drove here to pick it up. When he did, he asked about the vendor he purchased the diff from and if I would inspect the diff before he installed it.

He had it with him so I put it on the stand, pulled the cover, looked down, and told him it was junk and would not last long. He was kind of surprised, since it took all of 30 seconds to pull the cover and tell him it was junk. So, I apologized to him and asked him to look at it, then I took some caulk and outlined the 2" crack in the posi case.

This posi case would have imploded, depending on how the car was driven maybe 1000 miles, maybe 5000, but it would have failed and his $1800 which was worth a lot more back then than today would be wasted. By the time that happened any recourse would be lost. This diff was NEVER installed, this is exactly how it was shipped to the customer.

He was very upset, to be polite. He called and argued with the owner and was told something to the effect of "It's a 40-year-old diff, what do you expect" He ended up getting his money back, but I can tell you time and again where I read guys had failures from this place and they continue to be in business.

So, when I hear people say they are going to bring it to a shop that does vettes, or races vettes, I ask them to ask the questions I listed above. It doesn't take long to see through fake people in this business. If they polish and tune a posi, ask for pictures- very simple thing to do. Then there are some who claim to do this and put in about 15 minutes on it, saying it would take off too much material. Yes, if you don't know what you're doing that can be true, but the real reason is time and money.

Now today with all the selling off of smaller businesses, do you really think things will be better? Many places that once rebuilt units now get them from wholesalers or conmen.

I can spend the next 8 hours writing similar stories, like the place everyone thinks is great but never sees their work. They take worn out steering boxes, flip the worm nut upside down, and resell them. You buy them, they are junk. The place you bought them from has no clue since they just bought them wholesale from the rebuilder and resell them. After spending and wasting time and money you move on to a Borgeson box -without really knowing what a correctly built box would be like. I see the same for Trailing Arms, look at how many are for sale on ebay. Some out there have been scamming well before there was a WWW.

Good luck.

 

Lt. Mike
I get the impression you are in central or S. NJ somewhere. So am I.
If you want a hand or just another set of eyes I would be willing to come help.
There are so few good mechanics left for these old cars we need to help each other out.
I just retired yesterday so I've got the time.
I'm a serious amateur race car engineer and I rebuilt & blueprinted my diff with GTR1999's help & mentoring, so I know what to look for.
Leigh

 

Bottom line…..Gary Ramadei is the ONLY person or company to rebuild a differential today. It has been shown here that most of these Corvette vendors are failing to properly rebuild them. I did have Bairs build mine years ago and it’s still holding up, but that was five years ago.

Gary builds these to standard far better than others…..so make right choice or pay the consequences. And his prices are very reasonable and you get way more than your moneys worth. It’s a no brainer…..or just try others and find out the hard way.

 

WELL Congratulations!!!

 

Point taken and will consider it. The shop I’m speaking of has done work for me before and a lot of people I know. The fellow’s name is Dean and I’ve never heard of anyone who had a problem with his work. It’s all he does and is the go to shop on the Jersey shore. I’ll do my own inspection of the posi before I bring it and if theres the slightest doubt that it will hold up I’ll put all new internals into it, posi unit, gears, whatever it takes.

 

Thanks Leigh, I’ve done a lot on older cars myself having built engines and can do most that this car could need but setting gears up is a little outside my wheelhouse. I’ll remove the unit but let the pro rebuild it. Thinking of a lot with this and considering bumping the gear ratio from it’s 3.08 to a 3.36 set. Should still have highway manors but give it a little more zip.
Oh and you said you just retired so now you have time ? Good luck with that ! I’ve been retired 8 years and most everyone I know like me would agree that they are busier now then when they were working… and no one can explain it !

 

With serious noises inside, most likely all the rotating parts are junk. You don't want to use chipped gears and the posi housings crack.
I would recommend:

• Pull it if you are comfortable and open it up. Inspect the carnage. Decide if any rotating parts are re-useable.
• Go ahead and buy a brand new eaton posi unit. These posi units are marginal at best, and crack/split easily behind a BB. I found 4 cracked ones in a row when I did mine.
• Gut the posi unit and put solid disc clutch discs in it, not what it comes with.
• Put new gears in it, but the stuff that is available tends to make a little noise at times. They must be corvette gears, they are cut different.
• Make sure you put all new Timken bearings in it.
• Seriously consider grinding all the rough edges off the posi housing, round them off, and polishing it. It's only labor if you do it. The corners of the windows are a stress riser and many cracks start there. It seems like that is one of the weakest points in this diff.
• It is also worth your whole to tune the posi like Tom's Diffs video shows. The spider gears have a huge amount of slop when setup like factory does it, like .120". This creates shock loads on the forementioned crack prone posi housing. When the posi is tuned there is very little clearance in the spider gears, almost zero.
• If you are mechanically inclined, you can do the above. Then consider letting a pro setup the gear backlash. That is the one thing that is common to regular diffs.
• Do the above and you will have a unit much stronger than you could buy at any posi rebuild shop or corvette shop. They won't do the above steps. They most likely think they are not needed. But on a BB C3 they are.
• The other, easiest and best choice is to send it to GTR1999. He will / can do all the above and more. He custom machines it where needed and assembles a diff like a blue-printed engine. You will find none better and none that builds diffs at his level.
• After learning what Gary taught me about these diffs, I would never go to a regular diff shop with a C3. Ever. There are too many issues like the above that do not happen or are not necessary in regular diffs. But if you took them an assembled, prepared, polished, tuned posi unit, I would trust them to setup the gears.
• Then there is setting up the stub axles, but that is the last step and another story.

 

I put a second (third, fourth or fifth...) recommendation for getting the diff work done by Gary. He did mine and also my steering box. Top notch work with lots of detailed photos. Shipping cost from WA to his shop in CT wasn't cheap, but well worth it. Probably much less expensive from NJ with the option to get it there yourself since NJ isn't too far away from CT.

BTW, I have a brand new set of U.S. Gear 3.36s that I got for my rebuild but they weren't needed since Gary said my original gears were fine. I heard that U.S. Gear are hard to find now and are better than most, if not all others, now available. Let me know if you're interested...

Paul

 

Thanks Paul, I appreciate it. I’ll let you know.
Mike

 

Hey Mike,
Sorry to hear about your diff.
I have a rear differential specially shop that I have used on other projects over several decades.
These guys know their stuff.
When I talked to them about my Corvette differential they said, they could do it.
They had rebuilt a few over the years.
I asked them a few of the same questions Gary posted for you to ask your trusted mechanic.
They didn’t have a clue about what I was asking about and said they basically replace what needs to be replaced.
I ended up having Gary build my differential and I couldn’t be happier.
It was more expensive, but I now know it’s right and will hold together.
I was so happy with Gary’s work I had him build my trailing arms and my steering box.
I have hundreds of pictures Gary sent to me while he was doing his magic, plus pages and pages of his documentation regarding my builds.
The one thing about having Gary do your differential is, you’ll never refer to him as a trusted mechanic, you’ll refer to him as your good friend who built your differential.

 

I’ve had a full plate this week being pulled in multiple directions and little time to think about the Vette but it’s ok as it gave me time to clear my head.
After service today I put the car up on jackstands and got to work pulling the spare tire carrier down so I could get to the fill plug on the diff. I also gave all the universals a good inspection and they all look fine. With the plug removed I used a bore scope inspection camera to have a look inside.

Well its official, the rear is blown !

the posi units center section is gone! The right and left axles have no connection turning independently. The ring gear looks ok but it hasn’t been removed yet for a pro to inspect so …. I’m leaning heavily on changing the gear ratio from the 3.08 to a 3.36 set anyway. I’ll have it out in the next couple days and will be moving forward towards getting it rebuilt. Looking into which posi to go with. A lot seem to use the Yukon units but not sure what their quality is. Eaton should be good if its available. I remember Moroso Brute Strength being good back in the day (?). Then I’m hearing about the newer gears made today like Richmond hum loudly…
Lots to consider.



I have to assume the springs and a lot of other parts that surround the spider gears are lying at the bottom under the oil.


The silver 12ton jackstands in the back are all the way down. They give security and plenty of room under the car to work.

 

amazing pic.
I have looked for one.
I am not ignoring the broken bits but it sounds like you have it covered.
It would be great to follow along your repair.
god hunting.

 

The removal of the center section is not hard (complicated) but its not easy ( its about 100lb.s and I’m 62 with one heart attack on the books )
I’ll get it done, it’ll just take me a little longer. Then the unit will be sent out / dropped off for rebuild by a pro.