manchestershark2007-11-02 08:19:31

Was posi fluid used?  The local vette guru here says that he has been using 2 gm posi fluid bottles to get the clunk to stop.  He now uses K&W posi trac limited slip additive (Auto Zone carries it) and one tube did it for the three vettes I personally saw use it including mine.  I changed the u-joints (powder for needle bearings) and still had a clunk.  I had used 1 bottle of GM Posi and still had a noise so I changed it out for 1 of the K&W and had no sound.

Don't forget to check the cushion and bolt holding it at the front of the pumpkin.  It is probably in bad shape if it hasn't been changed.

Before I bought my blue '72 I heard that clunk in the rear, so I took it to another mechanic and we checked the u-joints, drive shafts, bearings, etc. and could not find anything wrong.  I bought the car anyway thinking that I'll wait for something to go out, then deal with it.  That was 34 years and 55,000 miles ago, and I'm still waiting for something to go out!!  Over the years, I have put in several posi-additives but not the K&W that dnv suggests.  Thanks for the tip, I think I'll that one this winter.

Dean  

I started a new thread for this, since it was off-topic for the thread it was in.

I agree with Tim on the clunk...it's prolly the front diff cushion, since the diff itself has already been gone thru. Posi nosies are normally more prevalent when turning corners, but not always. I've never used the "KW" lube either. I do use the Lubgard brand of posi additive, and it works great.

To:  Joel and Tim,

Thank you for the input on my '72 rear end clunk.  I know the grinding/growling sound the posi makes when hot and turning slowly.  The additive helps that.  How do you check the front cushion and what is involved in changing it??  Thanks again!

Dean

Visual inspection will tell you a lot.  Look to see if it is reaaly hard or cracked.  Also see if maybe the bolt through it is loose allowing it to move.

Tips or tricks? SUUUURE! Get someone ELSE to do it for ya!

Unbolt the lower bracket from the diff first. Taking the driveshaft out is really the easiest way to get to the nut on top of the cushion, but I've managed to change them without having to do it. It's tight, but it can be done. Just keep track of how you take the pieces out, as far as which one goes on top/bottom.

If you get the rear up in the air, you can usually move the front of the diff(nose) by hand, if the cushion is really sloppy/worn. If you see any of the rubber pooched out, or cracked, it's bad.

Just changed my own differential mount to a polyurethane piece.  Several of the big parts houses sell kits.  I believe early and late C3s are slightly different, but it wasn't a bad job.  Had a friend helping me... it would've been difficult with only two hands.  Anybody that can change that thing without removing the driveshaft is a better man (or woman) than me! PSU 982007-11-01 19:17:25

The alumnium "bat-wing" diffs are a bit different to change...they sit a little farther forward than the older cars, which makes it even tighter to get to. Plus, the front bracket doesn't un-bolt on them.