Z4 shift knobs - help needed

I'm having a shift knob custom made for a friend's Z4 - not sure of the year. I do not know if the OEM knobs are threaded [size?] or splined or ???. Any help would be greatly appreciated. Thanks.

Neither. There is a slot in the shift lever and a tab on the shift knob that lines up with it and then the knob slides on and 'clicks' into place.

Not the answer that I was hoping for, but thank you for taking the time to respond.

Here's a picture of what the shift lever itself looks like...

So how hard is it to replace a shift knob on a Z4?

I ask because the 'chrome' finish on my 6-speed's knob is wearing off/flaking off, and leaving some sharp little edges. I've actually gashed the palm of my hand a couple of times.

Figure I need to replace the knob, but didn't know if a beginner DIY could replace it or not.

Just pull hard straight up and it will pop off Quite easy actually! When you put the new one on, just make sure it clicks in place like bimmerdan mentioned.

GR