You didn't buy the wrong RSB, I had the exact same problem. When I contacted Progress they were a bit surprised, as was I! Here's how to resolve it....the spacers they give you in the kit, the ones that are meant for the R18, use those, you'll need to modify it, I know PITA, but when I did it, it stopped making noise. I regret not saying this earlier in my post before, if I had to do it all over again I would have liked if Progress was able to tell me ahead of time the issue. I had posted pictures of my issue, which looks identical to my issue on ********'s website.I have a 2013 Si Sedan... has anyone had clearance issues with the 24mm progress RSB? I have had it for a while but started hearing a clunking a few weeks ago. I thought it would have been the end links (I still have the progress end links) but they were not loose at all. I took them off and inspected them and they seem just fine. After a closer look, I see that I have almost no clearance at the middle of the sway bar and I can see that part of the finish on the sway bar has rubbed off. I think it is getting caught and that is what the popping sound is coming from. Any suggestions? These photos are of the suspension at ride height.
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It's a pain, I feel ya, I really do, it took me a while to figure out what to do about it. It has nothing to do with the car being lowered or anything like that. My 2012 Si was stock, I even disconnected and reconnected the R and L brackets for the RSB...in some magical attempt to work, it did nothing. If you can't modify the spacers I would use a few washers in between, like just enough to give you a few millimeters of space. The bolts back there are 14mm I believe. The issue is the bar is hitting the back of your car, where the LCA connect to the frame. I actually had it pretty bad because it took some of my paint off!
Good luck buddy