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.

 

 

1 hour ago, Jeff G 78 said:

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

Dang it! That's another tool added to my Amazon wish list.

3 minutes ago, SteveJ said:

Dang it! That's another tool added to my Amazon wish list.

Me too.  We have used them at work for years.  They used to be wired which was a bit of a pain near spinning components.  I'm curious if the commercial version and the consumer version give similar quality.  

From my records I used KYB 361001 and 361002 on my early 260z chassis. @Randalla you got a photo of the gland nut? Mine also did the same clucking sound and it was a loose nut allowing the strut casing to shake around in the strut tube. I've also experienced what Steve was talking about with a loose strut bar connector, but it was only over speed bumps.

There is a substantial difference in length between short tower (240Z) rear shocks and tall tower (280Z) rear shocks.  A short strut in the tall tower tube would let the gland nut crank all the way down and the insert clank around in the tube because it would be hanging free.  That would be an early insert in a late model tube.  You can tell early versus late by the height of the rear shock towers inside the car.

36001 is short and 36003 is tall.  You would not be able to fit the nut on a 36003 in a short tower strut tube, it would stick out of the tube too far.  

https://www.rockauto.com/en/catalog/nissan,1976,280z,2.8l+l6,1209226,suspension,strut,7584

https://www.rockauto.com/en/moreinfo.php?pk=502600&cc=1209226&pt=7584&jsn=702

https://www.rockauto.com/en/catalog/nissan,1973,240z

https://www.rockauto.com/en/moreinfo.php?pk=502428&cc=1209170&pt=7584&jsn=652

Look at the wheel well bump compared to the strut tower.

https://bringatrailer.com/listing/1972-datsun-240z-222/

https://bringatrailer.com/listing/1978-datsun-280z-119/

Edited February 19, 20223 yr by Zed Head
+ tube

Correct struts are ordered. I'll report back here on progress and results in a couple of weeks. Thanks everyone for the thoughtful replies.

One more idea. Kinda far fetched, but since we're throwing stuff out there...

There is supposed to be a spacer between the upper spring perch and the rubber insulator mount. On the front they use a bearing there so the front strut can spin when you turn the steering wheel. On the rear (since the rear doesn't have to spin) they use a hard plastic spacer instead.

I only bring this up because I don't see the spacer in your pics. Maybe it's stuck inside the rubber mount, or maybe someone lost it along the way?

Here's a pic of the upper insulator mounts and their respective bearings/spacers. Rear pair on top and front pair on bottom:

Good luck with the continued hunt for the noise.  

Nevermind... I think I see your plastic spacer stuck inside the rubber insulator mount:

17 hours ago, Randalla said:

Correct struts are ordered. I'll report back here on progress and results in a couple of weeks. Thanks everyone for the thoughtful replies.

Are you saying that he had the wrong ones installed?  You said that they were new in the first post and that it's an early model.  That would be short in short.  How can they be wrong?  Not really clear what's going on. 

The struts were recently replaced by someone else to try to solve the problem, but from the pictures posted on Rock Auto showing both early and late strut cartridges, it appears that person used the later inserts. When I take the strut apart next time I'll be able to verify if he has the correct KYB part number. If not I'll swap them. If he does have the correct part number I'll have to keep searching for the problem.

@Randalla did you get to the bottom of this?

We've made incremental improvements, but it still doesn't seem quite right, though I'd say it's maybe 50% better. Turns out the strut inserts were the correct ones for his car after all, and after seeing both versions in the flesh I don't really see any way they could be assembled incorrectly, as dimensionally they are quite different. Any improvements made can be attributed to three things I think. I added a small amount of gear oil in the strut tube before inserting the KYB inserts to neutralize any harmonics that could have been created. I also replaced the spacer between the spring perch and insulator, though it was only a small amount compressed/deformed. Tires on his car were set at 35 lbs., so I deflated them temporarily to 25 lbs. as an experiment. I did climb in the hatch with my stethoscope to see if I could isolate where the noise was coming from when driving over the road reflectors. Unfortunately that did not yield any new information. This is a moving target but we do seem to be moving in the right direction.   

People strap cameras under their cars and have good results in seeing what's happening.  And if you posted a video of the noise somebody might recognize it.  It's weird how the ear and brain can work together.  "Banging noise" covers a lot of spectrum.