Gerhard,
Your mechanic is probably correct. I can't speak for rotax, but almost all cam lobes are ground with a slight taper which forces the lifter to rotate as it runs. It is a very slight lateral taper which you can't see without measuring instruments but it is there. Again, I can't speak for rotax, but the lifters are often not flat either. They are ground from the center outward to match that very slight taper in the lobe to improve rotation of the lifter. Sometimes you can see that one edge of the cam lobe is shinier than the other edge due to the extra pressure exerted off center to aid rotation. Because the other end of the lobe is constantly being wiped clean, this does make deposits more evident in the center of the lobe. Don't know what genius figured all this out, but it works.
Bill.
