I just had my custom shift knob fitted, and thought I would share info about it here just in case someone else might be interested. This was made to measure for my Z4MC, but I'm sure it would fit almost any BMW with a stick shift.
This knob was taken off another car of mine (JDM make). It is made of delrin on the top half, and stainless steel on the bottom. Basically best of both worlds, a weighted shift knob that won't burn my palm/fingers on a hot day.
In essence, I needed something with set screws to secure on the shifter of my Z4M, that also had a male thread pitch of M12x1.25 on the top for my custom knob to screw on to. I spent a good amount of time searching the internet, but couldn't find any good solutions. So I went back to the guy that made the knob for me in the first place, and was given two options. He could either fabricate said adapter for me, or he could make another stainless steel bottom for me that would fit my shifter. I went with the latter because I felt it would give a more oem look, and also I didn't want to extend the whole set up by a further 2 inches, which was what would have happened if I used the adapter instead.
I must say I'm very happy with the results.
If you want something similar, go to
www.lathewerks.com and get in touch with Chris. He's a great guy and his work is consistently top notch.