Sorry - another rear-end clunk

[Roberts280Z]

1976 280Z - has not been driven in a decade or more, and is now nearing the end of a multi-year restoration.  Stock 4-speed and stock diff.  When I first tried driving around the block, I had a loud clunk under several circumstances, such as letting off and giving gas.  I've spent hours reading through this forum and I do understand that most such clunks are diff-mount related.  I've jacked up the rear and placed the outer edges of the lower control arms on jackstands.  The clunk is quite obvious when rotating either wheel back and forth.  You don't need to be violent about it to hear the clunk.  That rules out diff mounts.  When you rotate either wheel, the corresponding inner-most hub at the diff moves in lockstep with no slack.  That should rule out splines and CVs.  Also note that I did replace all bushings during some previous restoration (I am the original owner).

 

I can measure about 0.75" of back-and-forth rotation at the tire edge before I feel resistance or see the beginning of motion of either the drive shaft or the opposite wheel.  That's about 3.5 degrees of wheel rotation.  I don't know if that by itself is abnormal or not.  I don't think the clunk is normal.  The only comparison I have is to some old GM A-bodies.

 

So guys, does this indicate that the next step is to remove the diff for further investigation?  Thanks in advance!

[calZ]

Before removing the diff, I would check the u-joints on your driveshaft as well.

[Miles]

3 hours ago, calZ said:

Before removing the diff, I would check the u-joints on your driveshaft as well.

 

yep

[JMortensen]

 

[Roberts280Z]

Thanks for the replies.  I had indeed immobilized the drive shaft yoke at its input to the diff, so that should eliminate driveshaft u-joints.  And yes, I had read JM's definitive post several times.  But I do suspect that this is one of those 5% of situations where the diff is indeed the problem.  BTW, I did perform a similar 'wheel-rotation-slack' test on another car I happen to have currently on stands, and it had less that 1/4 of the slop I saw on the Z, and the other car has an LSD which I suspect adds even more slop.

 

So I assume that I've got excessive backlash either between ring and pinion, or side-gear to pinion-mate.  Or the pinion shaft is wallowing around in the differential case.  I'm sure I'll need more help once I get in there.

[Roberts280Z]

So yesterday I began to remove the diff, and while trying to wrestle the half-shafts off, I noticed that the diff would move.  Indeed one of the nuts holding the diff to the mustache bar was loose enough to allow the diff to move, and with an audible clunk, which is cause #5 in Jon M's authoritative post.  About twenty years ago, I did remove and reattach the mustache bar when I replaced the isolators, so I guess I didn't torque it back properly.  Note that this required much greater force than I had been exerting on the wheels in my prior clunk tests.  So I will just put the diff back and after I get my new shifter bushings from MSA (or whatever they call themselves now), I'll drive it around the block to see if the clunk is gone.

 

I am still curious about the total backlash that I can see from either side-flange while holding the opposite side-flange steady, with the half-shafts removed, and without visible movement of the companion flange.  There were some visible shavings on the drain plug when I removed it.  I don't know if some amount is expected.

 

I guess I'll worry about it if there is still a clunk.

[Miles]

I solved rear end clunking in my 72 240z by replacing al of the rubber bushings with polyurethane bushings including the mustache bar mounts and solid mounting (MSA mount)  the nose of the differential.  Of course the differential is still a little playful, but every Z I have owned had a "loose" differential that would clunk when engaging the clutch. I learned to engage the clutch smoothly. No problems in 20 years.

 

• SBC 350  v8
• Camaro WC T5 trans
• 78 280z r200 differential
• Chequered Flag stainless steel stub axles.  Replaced the stock stub axles because the wheel  mounting flanges were warped causing my rear 240sx caliper brake pads to rub on the rotors. Over heating rotors drove me nuts until I put a dial indicator gauge on the flanges and rotated the axle.  Evidently the flanges are soft and can become bent/warped just hitting a curb or a pothole.   Not a problem for drum brakes, but with disk brakes the rubbing can cause rotors to heat up.