Stock thread length is approx. 19mm IIRC - our lug nuts are short as we have that rotating cone too -
If you want, when I'm taking measurements with caliper later for that midrange speaker, I have a couple lug nuts on my work bench I can measure the length of thread section for you -
I always used to get lug bolts from
www.ezaccessory.com - their chrome bolts were great when I had my VW's and they also have 10.9 black bolts too - great pricing too compared to BMW bolts at $4-5 apiece