The problem with trying to answer that question is you have no idea what the conditions that pen will see. You have to always remember and I am beginning to sound like a broken record but wood is wood. It moves any wood can crack. You are anchoring a piece of wood to a piece of metal. That metal does not expand and contract the same as the wood. Put that pen in an extreme humidity or atmosphere that is a dramtic change it can crack instantly or may be subject to crack later. When pressing parts into the tube, the tighter or harder it takes to press them in the more chances of cracking because of the stress you put on such a thin diameter piece of wood.
You as about ebony. If you want dark black wood you can always dye maple which is a bit better and more stable wood but again no guarentees that it won't crack. Many segmenting turners use Gabon Ebony in their projects. Here are examples of my use of Gabon ebony. The holly and ebony segmented pen is one I still own and it sits on my desk all the time. It is not subject to drastic climate changes. There is no cracks in it. I used 2 coats of thin ca as a sealer and then 8 coats of med ca as a the finish. I also and this is very important, sealed the ends with thin ca so that it wicked into the grain. I think this is a step alot of people forget.
The ebony and aluminum pen is not in my possesion any more. That sold before I got it off the lathe so I do not know that condition. I also make small birdhouse ornaments and use ebony and holly alot for contrast and they have no cracks. Again though there is no stress and the climate is not drastic. The ones I have ever shipped, I have never gotten any return calls.
I know this does not answer your question but to me there is no answer. Just make sure you are using dry materials to start with and hand push the parts together to elliminate the pressure of the kits when assembling.
