[Tambohamilton]

Just ran across this part on realoem, never actually seen its part no before, though my car did come with them fitted;
51717180143
https://www.realoem.com/bmw/enUS/par...&q=51717180143

That's the 3mm thick steel plate that's fitted between the strut mount and the chassis on some cars, presumably to make small ride height corrections.

[The Nightman]

I think they are strut tower reinforcement plates, commonly found on early model years. Presumably BMW wanted to prevent all the strut tower issues found on the E46, but they probably learned it was unnecessary as later model years didn't have the plate anymore.

You can see them here: https://www.e90post.com/forums/showp...48&postcount=7

[The Nightman]

There's also a 20mm spacer #31306779671 included in the rough road package S815A

[tcphoto]

I remember transferring it over when I installed the Supreme Power Spring perches, I suspect that it's not good to forget it and it may eliminate vibrations from the suspension.

[Tambohamilton]

Originally Posted by tcphoto

I remember transferring it over when I installed the Supreme Power Spring perches, I suspect that it's not good to forget it and it may eliminate vibrations from the suspension.

I've removed them from my car (as well as other changes), and they've made zero detectable difference other than a few mm of ride height.

I mainly just find it weird that they're not listed next to the rough road spacers. I can't see that they reinforce anything, since they use the same bolts and don't spread the load from the strut mount at all.

Honestly I think they make up the height difference between the m-sport and non-sport strut mounts, so that the m-sport mounts can be used for heavier cars with non-sport suspension.