brockbenson

Hi All,

I've got a 1970 Vette which I rebuilt the suspension front and rear, and had the diff centre re-built. Now back on the road I am getting a really loud clunking sound from the back when I turn corners or even turn slightly left or right. It sounds like someone hammering on the floor at the back end of the car.

The diff centre has new GM oil and Posi Additive, at the correct level. I tried doing some figure eights to bed in the diff and get the oil around the clutches. But this has made no difference.

All the rear end bushes are new, new fibreglass leaf, new shocks, new poly diff mount, new uni-joints. I have checked everything over again, but cannot find what is causing it. It feels like something is either rubbing or binding as it rotates.

Has anyone had this issue and found the answer?

Thanks in advance, BB

ZZ383

Have you lifted the rear of the car and tried rocking the tires back and forth (Press top, pull bottom/ reverse procedure) looking for bearing play? There should be no play or it's your bearings/races. It is hard to properly set up Corvette rear ends because you must build the units from inside to out unlike the unitized assemblys used in many of today's cars. You must have the proper shims installed to have zero bearing play initially but the wheel must turn freely. Two things that I found invaluable when doing my bearings were a shim kit which contained about 5 shims per side in slowly ascending increments. The other was a corvette specific bearing set up tool the which allowed a mock up with each shim to get the proper bearing tolerance prior to slapping all that back together. The huge problem with not having the set up tool is, if you have it shimmed improperly, you have to destroy parts getting it back apart to try your next shim. This could be very costly. Don't be surprised if each wheel takes a slightly different shim to get proper clearance. My car had recently been done but the P.O. had just stuffed the biggest shims he could find in it. The bigger shim actually pushed the bearing further out. In just a few thousand miles the bearings were starting to eat themselves. Just a thought, hope that helps.

C3Dave

My 79 was making an odd clunking sound sometimes when going around a corner. Ended up being a bad body mount.

brockbenson

Thanks for the information ZZ383. I did check the trailing arms before I pulled the rear end apart. They didn't seem to have any play, but I will check them again now that I'm having this issue. I would be surprised even if was the bearings though, as the noise isn't a grinding sound that bad bearing make. This is loud and sharp, banging.

Lets see what tomorrow brings

BB

7T1vette

Sounds more like a bad U-joint.

capevettes

I would try another 4 oz bottle of GM posi lube and then do your figure 8's. If that doesn't help I would suspect a bad 1/2 shaft u-joint.

brockbenson

Hi All,

I will check the drive shaft universal joints again. I am not sure that more posi-lube will help. I have already put in the specified amount by GM.

Does anyone know, is the C2, 1966 diff setup different to the C3 onwards? The reason I ask, is when I had the rear end suspension apart, I found a date stamp on the diff centre that said 9.15.66 ?

Could this be a factor in my noise issues? Something not compatible?

Ideas anyone,

Cheers, BB

brockbenson

Does anyone have a thought on this?

Still having the clunk issue.

Cheers, BB

7T1vette

The drivetrain for the C2's and C3's is essentially the same. So, that vintage of differential should not be the problem. But, there could be some internal damage/wear on the Positraction components that could cause that clunking. Did you take the car to an open parking lot and do some low speed figure 8's in both directions after you put the posi additive in?

brockbenson

7T1vette, yes I did about 10 -12 figure eights. The sound didn't get any better than before I started this.

This is a louder clunking, much louder than I thought the posi would make considering it is housed within a heavy cast iron centre and had heavy diff oil around it.

The best I can describe it is as if someone is hammering on the chassis at the back every time a turn the wheel. You can even feel the clunk slightly through the seat of your pants.

I think I am going to have to start stripping parts off the rear end until I find something faulty.

63split63

[QUOTE=brockbenson;1579810235]Hi All,

I will check the drive shaft universal joints again. I am not sure that more posi-lube will help. I have already put in the specified amount by GM.

Try another bottle of GM posi lube and more fig 8s before you start taking things apart.
KISS Theory

Regards Bill

72downunder

We had the same/similar problem. A lot of grating and clunking when cornering or reversing. Tried new diff/posi oil still the same. Mechanic took the diff apart and found a half shaft circlip rolling around in the diff.

Before we bought it it had the rear suspension completely replaced, so one of the 'older' axle circlips must has fallen off into the diff.

Need less to say the diff internals were little worn so have replaced with new. No more grating noises coming from the rear end.

JoeMinnesota

Check the torque/lash on the front pinion. I replaced the pinion seal on my '70 last summer and after reassembly and fluid change AND U-joints had the same issues/sound you describe. It ended up that the front pinion nut had been UNDER-tightened (by ME) by a turn. When I drove the car and things settled in there was play in the front bearing. The rear made horrible noises on turns at low speed.

Jack the car up, get underneath and grab the rear of the main driveshaft where it meets the rear end and shake the end of the shaft up and down, as if you're checking the U-joint. If there is much or any play, I would suspect your front nut is under torqued and bearing loose. This caused some horrible noise out of mine and re-tightening to the proper setting had it sounding normal again. No problems since.

Sluefoot

I also had to end up putting TWO bottles of GM diff. fluid in mine to eliminate the noise

Wee

Took 2 bottles of additive and 140 weight oil to make my noise go away....

Brian

Tom454

Original post is January 21, 2012. Should be fixed by now? Maybe? Never know.