This set is exactly what you need to mount a Bobber seat on your frame. The seat springs can be placed over the pen (Ø12mm). It includes two pins that you can weld to the frame or can bolt through the supplied stainless steel M8 Allen bolts with round heads. They fit on a frame tube 1'' (Ø25, 4mm) but they are also easy to adjust to fit on a larger diameter tube. In addition, the rubber rings ensure that your paint is not damaged by the springs and the springs do not squeak. 

Material: Easy welding steel!