All of our chair mechanisms are made in the U.S.A. from quality materials and will last for decades.  We believed in repairing, restoring and reusing high quality office furniture.  Our “old style” chair mechanism has a front to rear hole length of 13-1/2″ and a side to side hole width of 6″.   The threaded post is an industry standard 1″ diameter and the bell is 4-1/16″ diameter covering the top of the 4 leg base hub plate.  The rails curve outwards at each end.