This office chair parts kit includes all parts you need under the seat. The heavy duty seat plate has a mounting hole pattern of 6" x 10.15" and rated for up to 320 pounds. Chrome plated 5" stroke cylinder. The base is 28" in diameter and is made of cast aluminum with brushed finish. Ideal as an upgrade or replacement for executive Tilt/Swivel style chairs. Casters have a 55mm diameter wheels and are ideal for carpeted areas. All items are new.