The raw material for building the frame is round steel DOM (Drawn over Mandrel) tube that is 1-1/2 inches outside diameter(OD) and 1/8 inch wall thickness. Where two tubes are butted together I insert a 1-1/4 inch OD steel tube and then weld the seam all around. The 1-1/4 inch tube slides perfectly inside the 1-1/2 inch tube and is used for support to keep the two ends aligned, but also as a backing material for when welding the seam. This works really well and is used in about 6 places on the frame.
As other materials are used I will try to keep a list, but don't expect it to be accurate.