Just finished drilling, weighing, installing, and testing lightened heavy duty clutch using the trial and error method.
Drilled twelve 1/4" dia. holes at outermost diameter area of clutch halves, reducing clutch weight by 25 grams total from 200 grams to 175 grams. Material drilled easy, appears to be a very fine grained cast iron.
Tachometer shows that the clutch now engages at 2,750 rpm from 2,300 rpm idle, this heavy clutch runs exceptionaly smooth on road testing. Wish the engagement was up over 3,000 rpm for a little more separation from idle speed. Will test ride for a period of time over various trail conditions before deciding if further modifications are necessary...............Don