I've recently fitted a replacement downpipe supplied by Motad to cylinder 4. After approx 400 miles, the pipe has taken on a brown tinge and there is mottling to the top of the pipe.
After contacting Motad, they have told me that is unavoidable due to the new pipe being stainless steel and the older replacement downpipes are mild steel.
Has anyone else had this problem? If this is a known "feature" of their pipes, I would advise anyone thinking about fitting them to bear this in mind as it completely ruins the look of pipework.