Very odd issue with ST rear sway bar on S5
 

Okay, I have had an odd popping, clunking and/or banging sound from under the rear of my '90 GXL from almost day one after all my rear suspension rebuild work back in June. Yes I know I should have check on it before now. I read where it maybe the bolt on the the upper sway bar link end being too long and coming into contact with the edge of the seam on the uni-body just above the top of the link end just behind where the brake line is routed. So today I decided to to get under the car and investigate for myself. I did it both using ramps and with a jack to see the difference with the suspension at the normal ride height and at full droop. What I found was even more disturbing. I have also recently noticed a very odd nervous handling behavior during right and left transitions that did not seem normal or consistent side to side.

I have a '90 GXL with Eibach springs, KBY AGX struts and ST 3/4" rear S5 sway bar with the stock S5 end links, along with ES poly bushings and DTSS eliminators. The rear camber is about -1.7 degrees due to the lower ride height.

On the driver's side, not only was the upper sway bar link end bolt contacting the seam on the uni-body as others have noted, but the sway bar itself has been contacting the hex nut on the lower end link bolt.

In the first picture you can see the where the sway bar has been rubbing the lower hex nut. In the second picture you can see the rounded edge of the hex nut itself.

So I then checked the passenger side and found the same wear issue on the sway bar, but as noted in the third picture those forces have resulted in a broken the low link bolt. Odd handling issued solved. Boy is that going to be a PITA to remove the broken bolt. Luckily at least I coated the threads with anti-seize when I had the bolt last out in June.

I think based on all this contact, I will try to find a clean stock clean S5 rear bar to replace the ST unit with.

Could the bar have been installed upside down?

I don't know, but you are the second person to mention that possibility. I thought I had tried it both ways during the installation process, but I guess I will be swapping it around again to see if maybe I screwed it up. Either that or I may try the slightly smaller RB rear sway bar.

Here are some photos from when I installed the ST rear bar. It is mounted upside down?

Well I was able to confirm that the 19mm ST rear bar for the S5 #51160 was in fact installed correctly. I finally got an extractor kit and removed the broken 10mm bolt form the trailer arm. I removed the ST 19mm bar and installed the Racing Beat 16mm S4/S5 bar #14104 again with the stock S5 links. All the issues are now gone. I have no idea why the shape of the ST specific S5 rear bar did not work correctly in my car with the stock S5 links while the RB bar works perfectly - other than it is a better design.