260Z Rear Suspension Banging

[Randalla]

I'm working on a customer's 260Z (1974 narrow bumper/earlier VIN). The car was brought to me after it was worked on by 2-3 other shops who couldn't solve the problem, finally throwing in the towel and giving up. I'm determined to solve this issue for my customer.

 

When the car is driven over a tar strip or road reflector there's a very loud, disturbing banging noise. The sound is definately in the rear, and happens whether it's the left or right tire that rolls over the bump. The car has a pair of recently installed KYB strut inserts and all new bushings through out the suspension, but the problem is no better than before being installed. Car also has a brand new rubber differential mount.

 

When I received the car my first step was a visual inspection, followed by tightening every nut and bolt in the rear end to assure nothing was missing or loose. The banging was still present. At that point I thought the strut insulator may have come apart causing metal to metal contact when the suspension was loaded and unloaded. I pulled one of the rear struts out and disassembled it to look for damage or perhaps a missing part or incorrect assembly. Nothing was readily apparent (see pictures).

 

After contacting KYB, I learned they had two distinct part numbers for strut replacements on a 260Z, one for early cars and one for late cars. Each one is dimensionally different, 12mm vs. 20mm, from the bottom of the threaded portion to the point where the shaft flares. Apparently the correct part number for this car is 361001 (the 20mm version), but I don't know if that is what's on the car or not. I also noticed while I had the strut apart that there is quite a gap (1/4”) between the shaft of the insert and the hole in the spring perch that it passes through. I thought there might be movement/contact when the suspension was loaded and unloaded that could cause the banging. There were also no washers around the strut cartridge anywhere, which I seem to remember on other Z cars I've worked on.

 

At this point, all this is just conjecture, but I'm hoping someone with a similar issue could shead some light on the mystery and suggest other potential causes.

 

 

[SteveJ]

FYI - This is a screen snip from Rockauto's website.

I know I had a noise one time when I didn't tighten the links properly on the sway bar. I haven't tried to see if this is possible, but could the sway bar be mounted upside down? Would it matter? Sorry, I won't have time to put my 260Z in the air until the weekend.

Another possibility - Go through the RA section of the 74 FSM. See what it says about tightening some bolts. If one was tightened in the air when it should have been tightened on the ground, it could be binding.

Were the new bushings rubber or poly? If poly, were they greased before install?

Just spit-balling some. I hope it gives you some inspiration.

While my springs aren't stock, the rest of the rear suspension is. Are there any photos I could take under the car to help you?

 

[Zed Head]

1 hour ago, Randalla said:

When the car is driven over a tar strip or road reflector there's a very loud, disturbing banging noise. The sound is definately in the rear, and happens whether it's the left or right tire that rolls over the bump.

That's a pretty low amplitude bump.  A tar strip?

I would look at the unexpected.  The spare tire, any loose parts in the spare tire well, parts in the tool bins behind the seats, the muffler and exhaust system, the gas tank,etc.  And the diff mounts through the mustache bar, they have been known to be loose even though the nuts are tight.  And the front diff mount.  Even the bumper could be bouncing on loose bolts.

[Patcon]

I would also look around for cracks on the chassis. At the strut towers or strut tower braces, spare tire well. Anywhere it could make a noise. I agree with Zed on the low threshold for the sound. Maybe get some one to drive you around in the hatch area to see if you could localize it

[Randalla]

Thanks guys, appreciate all the thoughts. A couple of more pieces of information. I didn't mention it in the first post that we checked the spare tire well and down inside the rear quarters thinking maybe something had fallen in and was bouncing around. The bushings, with the exception of the rubber diff mount, were all urethane but were replaced by someone else trying to track down the issue previous to my work. The new urethane bushings didn't make any difference. The sway bar links are tight and the bar is oriented properly. I did find one loose exhaust hanger, but tightening it didn't make any difference. I like the idea of a test ride in the hatch to listen closer to the area I believe the sound is resonating from. I will be trying that. U-joints are good and half shafts are tightly bolted. Gas tank is firmly secured. I've done a lot of work on S-30 chassis cars (probably well over 50) but never encountered this issue before. Thinking at this point someone may have installed the wrong KYB part number, but I'm  not sure yet exactly how they differ and what the effect would be of installing the wrong part. To your comments relative to the low amplitude of the bump, that's what make this so strange to me. Going over a big dip where the suspension is fully loaded and unloaded does not produce the banging. I'll soldier on, but am obviously open to other thoughts. Again, thanks for your responses thus far. I'll keep this thread current as I get more information. Thanks for the offer of taking pictures Steve J but everything under the car looks fine. The car has always been in CA or AZ so very little chance of any repair or rust.    

[Captain Obvious]

14 hours ago, Randalla said:

After contacting KYB, I learned they had two distinct part numbers for strut replacements on a 260Z, one for early cars and one for late cars. Each one is dimensionally different, 12mm vs. 20mm, from the bottom of the threaded portion to the point where the shaft flares. Apparently the correct part number for this car is 361001 (the 20mm version), but I don't know if that is what's on the car or not.

It is my understanding that the diameter of the strut tubes got bigger in the later years. From 50mm to 55mm OD and a corresponding increase in the ID as well. The length may have changed as well, but I don't know for sure. So if the ID of the strut tubes increased, I expect that KYB increased the diameter of the insert.

They also changed the height of the rear insulator mount that goes at the top of the strut assembly. The later mounts are taller than the earlier mounts. The mount in your pic appears to be the earlier shorter version.

So I don't know if it's possible to even fit the wrong insert... I suspect (because of the diameter change) that it's impossible to stuff a newer design insert into an older design housing, but the converse may be possible. It may be possible to fit an older design insert into a newer larger housing.

Than about the top insulators... I know the overall height changed, but I don't know if that height change resulted in a change in length of the necked down portion of the insert chromed rod. I can theorize that the taller insulator used a longer non-threaded portion of the shaft, while the shorter insulator needs a shorter non-threaded portion.

All that said... Here's my thoughts. If they fit a smaller diameter insert into a larger tube, it may be knocking around inside there. And if there is some sort of mismatch on the rear insulator mount, the threaded portion of the insert shaft may not be long enough. You may be bottoming out the nut on the threads before you get to the shoulder. In other words... The nut will get tight, but only because it's binding on the threads instead of tightening up against the shoulder like it should.

Bottom line? Make sure all the parts are matched for the car. An early thin bumper 260 should have the smaller strut tube and the shorter insulator on top of the strut assembly.

And I'm a little confused by this part - "I pulled one of the rear struts out and disassembled it to look for damage or perhaps a missing part or incorrect assembly.  Apparently the correct part number for this car is 361001 (the 20mm version), but I don't know if that is what's on the car or not."

If you took a strut assembly apart, then why don't know if the insert is the right one or not? Isn't it as simple as looking at the part number on the side of the insert?  

[SteveJ]

Yes, @Captain Obvious, the strut number is stamped on the housing toward the bottom.

[Randalla]

Thank you Captain Obvious for this. You're confirming where my head is currently at. Re: your inquiry, why I don't know the part number on the insert, I reassembled the strut and installed it back in the car before I had all the information I tracked down subsequently. Looks like I'm moving toward replacing both cartridges with the correct ones for an early 260Z, and hoping I have the correct insulators. I only wish I had a duplicate set of early and late 260 components so I could inspect and mic everything, side by side. Anyone out there have a 260 parts stash that's willing to take on that challenge?🤪

[Jeff G 78]

I've seen several times when replacing strut cartridges that the new gland nuts that came with the cartridges were too tall and the threads bottomed out prior to clamping the strut cartridge into the tube.  I had to use the old gland nuts and then there was no vertical gap between the cartridge and the inside of the nut.  With the spring off, the cartridge should not have any axial play.  If there is any play, it will bang.  With the spring installed, it's impossible to tell if there is play, although if you see a lot of exposed threads, chances are good this is your issue.

[Randalla]

Thank you Jeff. Will definitely check that during disassembly/reassembly with the new cartridges. The current condition is that the gland nut holds the cartridge tight with a couple of threads still showing.    

[Jeff G 78]

3 minutes ago, Randalla said:

Thank you Jeff. Will definitely check that during disassembly/reassembly with the new cartridges. The current condition is that the gland nut holds the cartridge tight with a couple of threads still showing.    

With only a few threads showing, I doubt that is the cause.  When I had the issue, I showed 3 - 4 threads.  

The next thing I'd check is simple play everywhere.  Put the car on jack stands and then put a floor jack under the tire.  Have a helper slowly jack the tire until the spring begins to deflect while you watch and feel the strut top mount.  There should be no noise or movement from full droop to the start of travel.  Repeat the same test and watch/feel the stabilizer bar end links, inner, and outer control arm pivots, and even the mustache bar to diff and to the body.  Chances are good, the issue is in the strut somewhere since that's what changed before and after the noise began. 

If that fails to provide answers, have someone drive the car over small bumps while you listen to a stethoscope with the end on the top nut of the strut.  You should be able to confirm if the noise is coming from the strut or elsewhere. If that too fails, Chassis Ear will be needed.  They are the same as a stethoscope, but are wireless and can be attached to various chassis parts so you can listen while you drive over the proper input.  The noise can be isolated to a specific component rather than knowing that it's coming from the rear of the car.  Chassis Ears are about $100 for a basic setup.  https://www.amazon.com/STEELMAN-06600-ChassisEAR-Electronic-Squeak/dp/B000IHIAES/ref=sr_1_5?crid=2K7L0ZJIEUAHF&keywords=chassis%2Bears&qid=1645206093&s=automotive&sprefix=chassis%2Bears%2Cautomotive%2C139&sr=1-5&th=1

[Captain Obvious]

2 hours ago, Randalla said:

I reassembled the strut and installed it back in the car before I had all the information I tracked down subsequently.

Gotcha, and hope you figure out what's going on.

As for the insulators, it's pretty easy to tell the difference between early and late. The difference is significant. Here's a pic I hoovered off the internets of the two side by side:

The one you showed above appears to be the early shorter version. And that would make sense for an early 260.