The reason for the weight difference between the two is the weight of the ball bearing I use and with EI a different hub is used that screws onto the threads on the end of the crank.The stock magneto rotor is not used either way..
The stock Zenoah prop hub is 2.80 inches in diameter..I make an aluminum hub to replace the stock mag rotor..I use 2 inch aluminum..I turn down the stock hub to 2 inches to match the hub and re locate the 2 bolts in the stock hub...
The EI hub has no ball bearing on the back and is turned down in the middle to 1.4 inches to save weight...I will make the next mechanical advance hub using the EI type parts and save an ounce or two, we'll see...
I use the ball bearing because when I started making ignitions everyone else copied A&M with their sometimes sloppy and sometimes tight rotating piece..They tried phenolic, aluminum, and Delrin, finally settling on Delrin and a somewhat loose fit...There is no play in a sealed ball bearing, and it will not get tight...
My conversions use a G10 epoxy board carb rotator block and no bell crank, eliminating the extra link and possible point of failure....The throttle servo connects directly to the carb shaft...