and a little mess with shoulder and tailstock.
Are they synonyms or shoulder is synonym of headstock?

A shoulder is a flat surface perpendicular to the axis,
adjacent to a smaller diameter, or any similar feature where
the part changes diameters. If it is at the end of a part,
it is usually just refered to as a face. In the case of
your part, it is the section adjacent to the diameter where
you are cutting your threads. Threads are much easier to cut
when you don't have to worry about where you stop and start
the cut because of the rest of the part getting in the way.
This is why some were suggesting you make the part in two
pieces.
--Glenn Lyford