Are the RD350 and RD400 front fork tubes the same?
No they are not. While they are similar sizes there are significant differences. I have never attempted to replace one with the other. I think that if you had the entire fork tube it might work but I am not sure. The bottom of the tubes are shaped a bit differently and I don't know how that would impact the operation of the unit. You could always try it and see but I would suggest you try both legs and not just one.
Lyn Garland
thanks!