i beleive it has nothing to do with gas flow adding silicon ramp, Shafts in the past and some still have equally as good gas flow shape machined in the steel as shape with silicon. Re easier on tooling its the more expensive cranks that have this easier on the tooling method and then silicon added, If its just easier on tooling all cranks would be made this way. I believe its done to reduce rotational weight to aid spooling. Martin.