Half of my model disappears when Preview is pressed. 
Also with Classic pressed, doesn’t seem right.
Hello Doug Jones
More information please. Pictures at the least of both your ZSphere model, and the resulting preview. “Doesn’t seem right” doesn’t help other people help you.
When working with zspheres, understand that the root ZSphere does not have geometry of its own, and must be capped off with ZSpheres on both ends.
