There will be no expansion or other issues with this stem. The stem shown in the image is made of African Blackwood.
African Blackwood is considered to be among the hardest and densest of woods in the world; indeed, among some 285 species tested, (including Lignum Vitae), Gabriel Janka originally found African Blackwood to be the very hardest.
Very good info, I was going to say that the Blackwood is some of the most dense wood on this Earth as well but also that there is quite a gap between the inner wood of the unit and the stem itself. It would have to do a hellova lot of swelling in order to get stuck and by that time if it did surely it would crack. Good stuff @steama!