Woolf Aircraft stocks elbows in standard sizes up to 5 1/2" outside diameter in stainless, aluminum, and mild steel. Special materials used can be stainless grades 316 and 321, Inconel, and aluminum types 5052 and 3003. Normally the centerline radius is a multiple of the tube outside diameter. They are referred to as 1D (one times the outside diameter), 2D, 3D. A tight bend is 1D for 90 degree bends and 1 1/2D for 180 degree bends, standard is 2D, and wide is 3D or more. However, actual bend dies in-house will determine the specific bend radius used.