Snapping lines and control points to each other

Using the move/rotate tool, I would like to know how to reassign the centerline or any point on an object so I can rotate it or have snap too any point I desire. Im currently working on a mock up model and have a valve that I highlight, move the move/rotate tool so the centerline is running directly through the valve, then went it is deselected and selected again, the centerline returns back to its original location thats not the centerline of the valve. How do I assign it so whenever its highlighted, the true centerline is recognized. Then, how do I make that object snap to a center line that intersects to another object so I can build between the two without missing when they finally connect?