rob75383

I have the annoying pleasure of having a mysterious >clunk< in my drivetrain.
I get it going when going from nuetral or park to either drive or reverse. I also can hear/ feel it when I hold it in low 1 or 2, hit the gas, let off >clunk<, then hit the gas again >clunk<.

I've replaced bushings, checked the tranny yoke, checked the rear IRS for loose parts (all recently rebuilt), and checked the U-joints in the driveshaft.

I've yet to pull the half-shafts and re-check them, and I'm guessing its either the u-joints that connect the halfshaft to the spindle flange, or its something in the differential.
I put all the u-joints in at home, except for the spindle flange ones. The flanges were warped, and I had my machine shop do it. They said it was a PITA to get the flanges straight enough to press the u-joints in. Those will be easy enough to check, once I find the time...

Just in case I'm wrong about the flanges, I wondering what I need to do to make sure there is no problems with my differential.

When I got my trade-in delivered by freight, I brought it to my 'vette buddy at the local autozone (he since has moved on to open up his own 'vette resto biz) and he saw the marking compound on the gears. He said the mesh was perfect. My machinist , who also owns a '75 with a real nice 408 in it, also said that the mesh looked good.
I am wondering about what can go wrong inside the diff??
I am worried about too much play between the ring and pinion, although the mesh pattern looked fine at the time of install. I am also worried about how they shimmed it.

I figure I can drain the rear, and see if I can spot the wear pattern, otherwise, I'll buy some marking compound, and then see the pattern.
In the past, I jacked up one side, and tried to roll the wheel forward and backward... from what I think I hapened, I could only rotate the wheel about a 16th of an inch or so, but I can't remember for sure.
I've read about getting a dial indicator and checking the backlash-- is that related to the wear pattern, or is it separate??

I modified the rear suspension, rebuilt the trans, swapped a new diff, and a host of misc. engine mods all at once, and from the first moment, I had the clunk.
The rear feels like it has been stiffened big time with the removal of the stock strut bars with the rubber bushings, and having them replaced with spherical rod ends. I also used the VB&P rear crossmember isolater disks, which I hear stiffen up the rear significantly.

So, what do I need to do to make sure my differential is in tip-top shape???
That way, I can look for my mystery noise elsewhere.

wombvette

WOW, Long question, hopefully short answer. About the only things that cause backlash in a rear are 1) U Joints 2) Gear lash 3) Snubber bushings.

You have changed the U joints, You can check the gears by locking the rear wheels and rotating the driveshaft back and fourth. You should feel a minor lash. Its not likely that because you have no other noises which usually go hand in hand. The most common problem is with the front ( snubber bushing), or the bracket.

Everyone has problems with the outer flange collapsing when changing U joints. HERE IS A HINT GUYS. Remove the companion flange and bolt them together while changing the joints. No more colapseee. :)

Bobchad

Have you replaced the front differential bushing? Notorious for making the clunk.

rob75383

Yeah, I replaced the rubber diff bushing with a new poly one.
The old rubber one was only 2 months old, and showed no problems.
Still clunking with the full set of new poly bushings for the diff, trans, and
engine mounts.
I remember seeing a tool for sale that preloads the flanges for easy u-joint service.
If the trick works, it'd be a hell of a lot cheaper to bolt the flanges back to back as was suggested here.

As for my long question... sorry, but I thought that I could save some unneeded basic questions by stating what had been replaced, rebuilt, modified, and what has already been inspected.


[Modified by '75 383 ElkGrove, 7:29 PM 9/20/2003]

Dk Met Grn 80 L82

I have the same clunk. It seems that when the idle is at around 600 it is not noticable, but when the idle is higher ........clunk. :mad

Ronny

I have just had my differential gone thru with new bearing and adjustments to get rid of a howl that was very noticeable. After driving it a couple hundred miles, a clunk developed just as you have described. I found the two long bolts that go thru the diff holding the bracket for the front bushing had loosened up ( probable were not tight enough to start with ). Retighted them up and no more clunk. :steering:

wombvette

Loose bolts or worn bolt holes in that bracket was a common problem early on. As time went by running changes in the bracket were made which more or less solved the problem. However it is possible.