1965 rearend cluncking

[Andy F.]

I just purchased my first corvette.....it's is in excellent condition....with one problem. After driving it for about 20 - 30 min I get a ...sometimes loader than other times.......a cluncking in the rearend after I stop and then turn, right around the tires.....left turn, left side cluncks....right turn, right side cluncks.............took it to a chevy dealer who said it is the positrac........anyone have a suggestion as to what it might be?
Thanks

[David B.]

Re: 1965 rearend cluncking

Andy,
It may just be that the posi hasn't been driven much lately. If so, it can exhibit those symptoms. Try taking it to a parking lot and do a bunch of figure 8's with it and see if it goes away. If not, there is a posi additive that you can use and then do the same exercise. I doubt if you have a serious problem.
Dave

[Michael W.]

Re: 1965 rearend cluncking

Andy, David is right seems like its the posi indeed.....I had similar experience.

[Andy F.]

Re: 1965 rearend cluncking

David....thanks for the reply..........I will try it and see if it works.....the Chevey dealer did put the additive in....but still clunks and very annoying. If you see some nut in a red vette doing figure eights in a parking lot tonight...it's me.
Andy

[David B.]

Re: 1965 rearend cluncking

Andy,

Lol, good luck, would be interested in you posting the result!

Dave

[Tim E.]

Re: 1965 rearend cluncking

Andy - I had something very similar in my '66 which turned out to be the positraction clutch pack needed replacement. in my case, the noise was a very distinct popping, snapping, clunking noise that felt like something was bound up and then violently let loose. Like you, mine only occured from a stop and then initiating a turn.

In the process of troubleshooting I heard of a lot of other things to rule out prior to changing the clutch pack. You may want to rule these out as well: loose bolts that attach the half shaft flange to the spindle shaft, u-joints in either half shaft or driveshaft, stuck brake caliper pistons, loose rear wheel bearings, and (as mentioned earlier) posi additive and exercising the clutch pack doing figure 8's. Tim

[Edward M.]

Re: 1965 rearend cluncking

Just went through this with my wife's 79, and the same thing with a 72 before that.

Get the old fluid out of the rear end (use a cheap suction gun). Then add two bottles of GM posi additive (get the GM stuff, not the aftermarket stuff). Then fill to capacity with new gear oil.

Now go to the parking lot and do the figure 8's.

Worked for me both times.

[Tom S.]

Re: 1965 rearend cluncking

I agree with Ed. Drain the rear and put in 2 bottles of G.M. additive and top off with a good quality gear oil. Then go do your figure eights. I am willing to bet this will cure it.Good luck Tom

[Michael H.]

Re: 1965 rearend cluncking

Just went through this with my wife's 79, and the same thing with a 72 before that.

Get the old fluid out of the rear end (use a cheap suction gun). Then add two bottles of GM posi additive (get the GM stuff, not the aftermarket stuff). Then fill to capacity with new gear oil.

Now go to the parking lot and do the figure 8's.

Worked for me both times.

Yup, the best way is to drain and refill with the correct gear oil.

Other than a drain/refill, I would add TWO bottles of the special positraction additive. It takes quite a few miles for this stuff to mix and actually work it's way between the clutch plates so don't get excited if you don't get results in the first 15 or 20 miles.

VERY common problem. There are a lot of major differential repairs performed that are not necessary because of this issue.

[William D.]

Re: 1965 rearend cluncking

I heard the same noise this spring. I would hear the noise when first starting out, turning Lt or Rt. Turned out to be posi clutches were bad. Check if you have any extra play in the half shafts. Good Luck!

[Clem Z.]

Re: 1965 rearend cluncking

i would recommend AMSOIL rear gear lube along with the GM additive. from past experiences i have found that AMSOIL gear lube works the best in posi diffs.

[Gary R.]

Re: 1965 rearend cluncking

It's a common problem, clutch hammer. Try the gear oil drain(vacuum it out) add GM additive and 90 wt or 85-140 Lucasr gear oil. Drive and see if that does it. If not then it's the setup, in which case you can live with it or rebuild the posi. You have some choices of clutches if you do from the NOS snowflake and new fiber coated to the tried and true solid steels. The solids are the best bet, the others don't hold up as well.
Clutch setup is the key here, in stock form they hammer. Custom set they do not hammer and last longer.
Good luck hope the oil change does the trick.

[Andy F.]

Re: 1965 rearend cluncking

Thanks to all of the responses. I did try the figure 8's as I was told that my Chevy dealer did put an additive in...it didnt work..... I am going to start over again ....drain it, add it and try it. I will update

[Andy F.]

Re: 1965 rearend cluncking

Dave..........I did as you suggested........the problem is pretty much gone.......not 100% but much better. I have also learned to kind of make turns while not in gear..that helps too.
Thanks for your help

[David B.]

Re: 1965 rearend cluncking

Andy,

I am real happy for you that it was not a costly repair. It will continue to improve with exercise I think. Thanks for letting me know the result.

Dave B.