This is a really common problem with B&O stereo microphones. They snap in half! At the time of writing I have over ten broken BM5s in my inventory and some repairs for customers, and so clearly something needs to be done.
The rotating mechanisms were (usually*) made from plastic. With time and use, the plastic parts become brittle and the teeth break away. First the mic becomes wobbly and then can break away completely. These parts are also make the top part of the microphone difficult to service because it can be impossible to remove the collet without causing further damage.
And so I commissioned some replacement collets and can now offer repairs for this problem.
The replacement part is machined from brass and won’t break easily. There is some re-wiring to be done. The BM5 used a 9 pin socket which was not easy to replicate. And so we simply run the wires through a hole in the collet and solder together. The socket can get oxidised and noisy anyway, and there is no real need to remove the top part in general use.
Using a brass collet has the additional benefit of making a good electrical contact between the top and bottom mic, which means better grounding.**
Here is the repaired rotating mechanism which should be good for another few decades of use.
* Some later BM5s used a different design with steel parts. B&O clearly realised that they had a problem.