Thanks John 998, 2 more privets since last post , makes 22. Bill T. , Phil I. removed the half moons where there is no reduction in the drum strength. The weight was not removed to lower the weight of the bike, but to lower the static weight of a accelerating mass. The weight removal is at the most effective point of a round mass,( the outer diameter). One other consideration is the expansion distortion of the clutch drum friction surface. The mass area is the greatest at the transition area where the clutch drum backing plate is connected to the drum outer donut.
This is where mass expands in proportion , this causes the inner portion of the clutch friction surface to have a greater diameter (when subjected to heat) then the outer diameter of the friction surface . The half moons mass removal helps to prevent distortion of the clutch drum friction surface.This is one reason a clutch is more prone to slipping when hot, this is just one reason not all.
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