Hello guy's just became a member I like the site already, Bought a '79 last year, enjoy every min. of it.

 

So you have diff chatter? you need the GM posi additive. You will love this forum great guys and great information.

 

You need some GM Posi additive in the rear end.

It's probably due for a change anyway, so get yer suction pump, 2 quarts of your favorite gear juice, and 1 or 2 bottles of GM Posi Additive and have a Corvette afternoon!

 

Welcome, and just drain the grease and add the proper amout of POSI-FLUID and then a bottle or two of the G.M. POSI-ADDITIVE (ONLY). Once you do that, take the car and do some tight circles, left and right, and it should clear up,,,,,unless the rear is wasted to begin with. Good Luck,,,,Peace,, Moosie

 

As the guys said try the additive and fresh oil. If that doesn't work you can live with it and fix it but you'll have to drop the diff out. A lot of work but if there are a lot of miles you may have some other issues like worn yokes.

Change the oil and see what you have. If you need help I can build you a posi or tell you how to build it, that will not chatter or hammer.

 

So how much is a 3:73 unit with heavy yokes? I have no core to send.

 

Seems to be when it heats up a bit before the chatter but I will change with

1963-82 Rear End Lube Kits:

GM Axle Lubricant (3 qt.), formulated for use specifically on GM axles.

GM Positraction Additive (4oz.), when used in limited slip equipped rear axles, modifies excess clutch plate friction and facilitates applying/releasing of clutch plates.

1 Gasket



Is this true or does it matter? Lube then additive then lube again when filling?

Thanks again to all that responded and I AM going to have a vette day!

 

Cost is dependent on how you want it built. I generally do not do the typical advertised rebuilds. I just finished one up and put 25 hours into it, 5 hours cleaning and prepping alone. If you are interested just PM me and I'll see what I can do to help you.

 

You don't need that kit. First off you're not going to pull the cover off as you will have to drop the diff to do that. You have to suck out as much of the old oil as possible, then add 2 bottles of additive and use Lucas 85-140 gear oil, from any autozone or advance auto. Then do the figure 8's. This may or may not work. If not the posi clutches need to be setup properly, in fact they should be replaced with better clutches then the type you have in there now. This is a much bigger job and costs well over a $1200 to rebuild it right. Try the oil change and if it works enjoy the car, the warm weather is coming!

 

Thanks GTR1999, I'll try the lube first. hit 50 degrees here today.

 

Yes, the chatter tends to come after the posi is heated up. I first noticed mine after coming off the freeway - that first sharp right turn after the off ramp.

I got much easier access to the diff plug by removing the rear passenger side tire. Some like to remove the spare tire carrier as well.

You need some kind of suction pump with a hose attached to suck out the old stuff. There is no drain plug, only one hole near the top.

After you suck the old stuff out, put the posi additive in first for a very simple reason. If you put the gear oil in first, you may not leave enough room for the posi additive!!

I used the GM gear oil like you have in the picture above, and the GM posi additive, which does not have the word "posi" on it. It is called, limited slip differential additive, I think.

As stated, figure 8s after the job is done will work everything in and should eliminate the hot diff chatter.

 

Sabbath, I have the same problem with my 79. In fact mine is also black with red interior. I have tried draining and refilling mine, but unfortunately it didn't help. I hope you have better luck.

 

My 77 did this for a while, I drained the old lube and filled with Amsoil sever duty gear lube no additive and it worked great, no more chatter.

Neal

 

ok since I've been a lifelong Packer fan I type up some info for you here.
This can also be found in my rebuild post.

This is what you have.




You can see the plates and springs in that stock setup above. They are in constant load against the side gears and clutches. The spec on the spider backlash is .000-.008" ( same as rear bearign endplay) If they are set loose then the posi will be the same. If they are set tight then the posi will bind and chatter. The inner clutches will not slip around a corner. The springs keeps the load there. You will have posi but drivablilty suffers.


Here is a side gear with new clutches I just setup last week. There is not a lot of room there and when the oil gets old and thick they grab too much with the spring force.





Here is the current offering for new loaded cases. Look at the springs in there along with fiber clutches, 1 less per side too. They fail under heavy load, one I had smudged in my hand- brand new. You should use 2 bottles with this setup.





This is one of my favorites- those snowflakes out of a used posi.A gallon of additive and 48,000 figure 8's aren't going to help this baby.





This is how I set them up, no springs, backlash tuned in. It has posi, turns corners without hammering, less load on the clutches, no snowflakes, no fibers, 10-17 spiders.




OK posi 101 is over for tonight, get ready for mid-terms!

 

wow thanks GTR1999 that loaded casing looks very nice! and clean! thanks for all the info that you supplied really helps me understand what the heck is going on in there.
And to all others thanks for the tid bits really helps.

GOOOO! Packers! (mabey A better year this time)

 

I went to get some juices for my '79 went to true value and bought a drill pump (works very well!) and heated up the lube in the rear end - went to the garage and started sucking out the old 80w-90 lube OMG it was Black! put in 4oz of limited slip lube+2qts of new golden brown lube did my 8's and walla no chattering or knocking what a relife that was.
and again guys thanks for the help be talking to you guys soon again.