I MAY have discovered something today...

[J!m]

Going through my Group N parts lists and also trawling the realoem site I think I made a connection, with the help of Turner.

Take a look at these part numbers and descriptions. The OD thread of the PSS9 from Bilstein is M52X1.5 (Turner confirmed this for me). But the PSS9 uses springs from Bilstein (Eibach?) which have a very small lower diameter, similar to a coil-over (probably for tire clearance) and the top is standard diameter, to fit in the stock upper perch.

However, looking through realoem, there is a "spring pocket" to only be used in conjunction with "threaded ring M52X1.5". I believe that this spring pocket is the lower spring perch and the threaded ring is used to secure the perch to the PSS9 strut.

Left Strut, M3 (Group N) adjustable* 31.31.2.501.111 1 $976.50
Right Strut, M3 (Group N) adjustable* 31.31.2.501.112 1 $976.50
*POSSIBLY Bilstein PSS9 set up for stock springs, which would be used with the following:
Spring Pocket (lower perch) 31.33.2.222.879 2 $110.94
Threaded Ring (M52X1.5- same as PSS9) 31.33.2.200.014 2 $72.65

The diagram shows the "spring pocket" as the rubber isolator for the spring in the standard strut, so it is confusing...

Turner could not confirm the "spring pocket" is in fact the lower perch. Buying these (non-returnable) parts to test them seems risky, but what do you guys think?

I had asked Turner prior on the BMW part numbers for the struts t see if they crossed to the PSS9 and Turner could not find any information on that unfortunately. If they could, that would put it to bed I think.

[MParallel]

I’m confused. What exactly is the discovery? Some style nut used by more than one manufacturer?

And PSS9 using Bilstein springs (Eibach)? PSS9=Bilstein so no idea what you’re on about.

[J!m]

The BMW Group N adjustable struts.

I think these are in fact the Bilstein PSS9 struts (Only) without the Bilstein supplied springs. The lower perch is a BMW part, as is the lower perch lock nut. Threads for the PSS9 and the BMW lock nut are the same. Changing the lower perch allows stock BMW springs to be used (same diameter top and bottom).

nd the Group N and Bilstein are both 9-position adjustable (values for each position are known for the BMW part, but I can't find them for the Bilstein part). Having those values match, would be a pretty solid confirmation. Turner couldn't confirm more than the threads matching for me.

[MParallel]

My PSS9's already use same diameter springs. Maybe the stock spring rates aren't compatible with the shock specs. I don't know.

It could be the Group N is that. If it's not in-house, BMW will also look at other brands.

[J!m]

Current PSS9 have a smaller diameter to the bottom of the spring (tapered spring), and the BMW part is (presumably) to allow the required stock springs, so a spring diameter at the bottom that matches the top.

I have seen the paperwork that goes with the PSS9; do you have any documentation that says what the compression and rebound forces actually are for each position? That is information I cannot find, so that I could match it against what BMW published in the Group N part list.