Office Chair Parts Bottom Plate, Cylinder, Base, 5 Casters Complete Under Seat Kit.  This office chair parts kit includes all parts you need under the seat.  This Kit rated for load capacity of 450 pounds. This kit includes:

1 SP610-SHD Seat Plate, 6" X 10.15" Mounting holes rated 500 lbs
1 SHD-GC4 Pneumatic Gas Cylinder rated 500 lbs
1 HDCB-P28 Base rated 450 lbs
5 CH-65 Casters rated 110 each for total of 550 per set of 5