Thanks Brian - didn't know that but it makes sense. We tried to create a part in the context of the assembly and everything was OK until we tried to select the datum curve as the trajectory - and idea why we couldn't select it proper or from the model tree?
You can create a part. In 2001 you have to create default datums, or I use csys, (pick the assy csys). Something like advanved/swept blend/datum curve/setect all, default orientations, sketch the circle on both ends. It will update when you change the curve.