Tbe America, Bonneville and T100 all share the same 17 tooth sprocket. The Speedmaster has a 16. I don't know what the Thruxton has, but I would think the 16 is likely. You can get an 18 or 19 here
http://www.newspeedmaster.com/html/18_tooth_sprocket.html but you'd probably only like the 19 if you had the big bore kit in your engine or lived in a very flat area. There are usually a few 16's available on the classified section of these forums.
