Hey Guys,

Here we go, I acquired my dream 69 convertible last week and although we cruised it all weekend and had fun, it’s going to need some work.

I had the car inspected and the mechanic stated the rear diff was totally shot. We had it out and it whines and I get a nasty vibration out of it between 55-60 mph.

I have no issue dropping the housing and I believe with a buddy (also a C3 guy) we could install the new diff. Should I purchase a new one? Ship off for rebuild?

I am located in St Louis, any leads on a local shop for this or the tranny us appreciate as I am grinding just a touch going into 3rd and want to address that as well.

Thanks for any and all of your input guys!

 

Shudder, cringe, shivers.
Every time I read the statement: My mechanic says to blah-blah-blah.
Sorry. But usually true.

This issue could be as simple as a flush of the rear axle fluid, adding the "correct lube".
It could be as simple as some bad U-Joints.

If you have never dropped a C3 differential, do not believe it's a simple job like other cars.

The grinding issue could be just a clutch rod adjustment. IDK

Standby. This forum has a differential expert. Wait and see what he says.

 

Welcome to the Forum.

Agree with @HeadsU.P. . Telling you something is “shot” is shorthand for, “I have found an excuse to charge this guy a lot of money”.

Start by changing the differential fluid. Get two bottles of Delco 10-4003 additive and Lucas 85w-140 gear lube. You will need to siphon out the old fluid as there is no drain on the differential.

Are the U-joints greaseable? If they are, grease them. You’ve got nothing to lose here. Carefully inspect them. Know that the joints will bind on full suspension travel so you need to raise the suspension about an inch or so to allow the shafts to turn free.

With the car properly jacked, grab each rear wheel and “rock” it between 9 and 3 and also between 12 and 6. No rock between 9 and 3 is good, between 12 and 6 could be bad wheel bearing, or worn stub axles. Have another person look at the stub shaft flange at the differential while you rock the wheel.

As for the transmission, also first change the fluid. Get some Penngrade1 “Classic” gear oil 85w90. It is specifically formulated for “yellow metal” (synchronizers) which you have in the Muncie.

As you will see by others responding, rebuilding these differentials properly can be a challenge. Gary Ramadei is the resident expert on them so heed his advice. Be very, very careful if you decide to go down the path of replacing the unit as you never know the pedigree of it and you could be stuck with a far less capable differential.

Check the stampings on the bottom of the main housing at the split line to the cover and post a picture so we can see if it is original to the car.

 

69L88 makes a good point about U-joint inspections.

Normally with other cars, you just jack up the rear and start turning tires.
But with Independent Rear Suspension, the tires will drop enough to cause binding.
Remember, you have six U-Joints. You need to grab shafts and start checking for unusual movement.
Have a helper turn the tires while you observe. Check with the vehicle in gear first. Then again in neutral.
Sometimes the needle bearing drop out of the U-Joint and vibrate at speed. Again, six joints, three shafts to check.

There is also a bushing at the transmission tail-shaft housing. But they seldom fail.

All rear axle fluids are available through Amazon. Just make sure of the correct GM part number on the bottles.
If you change the fluid yourself, its challenging to get at the fill port plug, but doable. Just have to use the correct wrench.
And no, you do not have to remove the exhaust or the spare tire carrier.

 

You need to determine if the diff is the cause of the vibration, typically it is not unless it is truly on its last legs. Vibrations can be from several things from old tires, alignment, driveshaft balance & angle , motor mounts, trans mount, bad 1/2 shaft joints, etc.

You have a new to you 55-year-old car that probably has been worked on my many incompetent people over the years. Blindly removing the diff for an exchange unit may lead to more issues, but maybe not.

Your mechanic may be versed in cars but possibly not too much with old vettes. Most on an old vette is standard fair and a good mechanic should be able to handle it, many are not into the specific unit repair however. It would not be uncommon for a local garage to condemn a diff, pull it and call the local parts store for a replacement. The parts store will go to their distributor for an exchange unit or the wiser ones will just call any vette vendor for an exchange. There won't be any detail work or information, those selling them rarely know how they are built and even if you did speak to someone who did, the standard answer is "rebuilt to stock dimension" about as clear as fog. They are a profit part, done fast and cheap for the most part. Most so called rebuilt diff's are nothing more than a new bearing setup. I just did a 74 diff that was rebuild 20 years ago by a vette shop. The same issues as you were experienced. They removed the diff and brought to me, I took it apart with the guy standing next to me. I found one bearing replaced and a cheap Chinese gear set installed. All the rest of the parts were the originals and worn out. The vette shop charged him more for that junk rip-off work then I did 20 years later to correctly build it. Today there are no more USA made gears, they are Chinese or Italian, they are better than the junk that was used 20 years ago but USA made gears were around 20 years ago and should have been used. Again- profit comes into view over quality.

You are wise to ask questions now. A word of advice, and I get it, you want to drive the car now and don't want to wait. Summer is coming up. Well that's where many end up losing more time and a lot of money and do it over again. I could post pages of pictures of bad rebuilt diff's I was shipped to correctly repair. Those won't help you since you don't know what is going on in yours. What I would do is remove the diff, mount it on a stand, pull the cover, drain the oil and see what you have going on.

The fastest thing is to change out the oil and use 2 bottle of GM additive and drive the car. If your issue is posi chatter, a common issue, this will usually resolve it. You don't have to pull the diff to do this. You just need the simple suction gun from NAPA , online suppliers, or Harbor Freight to remove the oil from the diff and add it. If that doesn't work and you checked the other things, then remove the diff with your buddy and check it.

I build diff's for some guys in the St Louis area, possibly they are here and see this to offer any advice on local shops.

Remember the most expensive & best tool you have is in your head. Use it correctly and you will save time, money, and lot of aggravation.

 

I blew up 2 rear ends and they never whined or gave me a vibration...I think it may be 2 different issues. Personally I like to exhaust the easy stuff first before blowing a couple grand when a couple hundred would have done it.

I vote for changing the fluid, checking the year dates on the tires ( replace anything older than 8 years old), gettingbthem rebalanced. Then jacking it up and blocking the trailing arms so the rear 1/2 shafts are level and testing the u joints. That way your buddy can rotate the wheels and you can feel for grinding. A vibration at speed would actually turn me away from the diff

 

Great info guys! Thanks! I have a 72, 350/383, 4 speed. 17 years ago I rebuilt everything EXCEPT the differential. Also 17 years ago, differential was "duck walking" in turns. Changing oil and adding traction additive solved it. Differential is "duck walking" again in turns. Chaning oil and adding traction additive hasn't helped. Is there anything else I can do besides pulling differential and having it rebuilt? A local mechanic told me "the clutches are stuck", just rev it up and dump the clutch a few times, that should take care of it". ??? I'm thinking that's the last thing I should do now. If I continue to drive it, just to local cruise ins', will it continue to get worse? Will differential stop working and leave me stranded? First time I've asked a question on Corvette Forum, used it for research many times. Thanks!

 

After you done slow “figure 8s” after changing the fluid?

 

Sounds like your guy is an old school mechanic, it won't resolve your issues, but you might enjoy it or break the diff depending on the condition. Don't use him to work on your car.

You have an original 1972 differential with who knows how many miles or abuse on it. 72's had the best cases and spiders, junk clutches, issues at times with RG bolts backing out and poor axles, well the axles were good, they just weren't hardened like the C2 axles were. I don't know what duck walking means, the rear of the car swaying? Chatter in turns?
Did you check the end play in the axles, make sure they didn't wear down and crash into the housing and seal?
No one can tell you if it will break or not since it would have to be inspected to know. If the posi is cracked and you're getting noise or whine out of it, or hear a clicking sound, it won't last long.

 

"Duck Walking" is a term I've heard describing a shaking feeling from rear when going around corners. I did figure 8s after changing the oil, no help. No leaks so no crack in housing. Right now I'm thinking just drive it to local cruise ins then pull it this fall. I'm close to Zip Corvette in Va. They say they can rebuild with new gears. I have not checked axel play. Thanks for your comments!

 

good luck

 

Personally I think you should contact someone that rebuilds rearends for a living, not a place that sells parts that has some guy in a back room just throwing a rearend together....maybe someone that is well respected and has a following on many different forums not just here and is talked about in magazines....sounds like a much better idea to me...theres this guy in Connecticut thats been doing it for years...you should ask around if anyone has heard about him or just type his name into the google machine......its gary ramadei.......he might have already given you some excellent advice, I dont know, Im kinda new here too

 

OP, Gary (GTR1999) won't say it because he's a very humble and helpful forum member - but I'll say it. If you determine that your diff needs rebuilding arrange a time and send it to Gary. If you've used this forum much for research, you'll note that he has put up several articles on diffs to help those who rebuild them or to know what to look for when having them rebuilt. If you want it done correctly, send it to Gary. He is to diffs and steering boxes what Lars is to Qjets and dist/timing. When you buy quality, you only wince once. Best, Paul

 

Scott, thanks for the good one!!

 

Thanks for the advice guys! Going to change the fluid, add the additive and check the U Joints before the weekend. Tires are 10 years old but appear in good shape, i'm definitely going to replace them sooner rather than later.

On the 3rd gear issue, going to change my tranny fluid out with some of the Penngrade and check my shift linkages to start

Will update with how it turns out!

 

Turns out it WAS NOT the differential! The bracket that the camber adjustment rods mount to under the differential had come loose! All 4 bolts that hold the bracket were gone! Replaced the bolts, had alignment checked, problem solved! I almost spent a bunch of $$ rebuilding differential but turned out to be an easy fix! Check the simple stuff first!

 

GLad you investigated further and didn't waste a lot of money

 

Makes one wonder what happened between the initial “inspection” and when your “mechanic” told you it was “shot”…..

 

It's mostly unknown but, in the late 60's GM was experimenting with 'self-correcting' sub-assemblies. It's apparent that the OP's car got one of these sub-assemblies.

 

sounds like the mechanic smelled some free cash