Unthreaded posts (spindles) at the hub entry point greatly eliminate Hub Liner wear and can offer greater stability. This is even more true when a chair is adjusted to be near or at it's maximum height range leaving little support in the center leg hub and liner.
Several inches of the post must remain in the center hub in order to give proper support when in use. Due to the leverage factor when leaning, the pressure of the steel threaded post threads can cut into the soft plastic hub liner. This creates a looseness and therefore the chair "rocks" annoyingly.
An unthreaded post end (3") cannot rub cut and enlarge the hub liner because it is machined smooth where it comes in contact with the hub liner.
Original installation posts were not intended to be removed and unless worn loose, can be difficult to do so. Before ordering any post, first make certain the old post can be removed. Notice the relative position of the keyway on the post. It is important when replacing a post for the keyway to be facing the left when sitting in chair.