As I look at the different examples the members have posted I think I see the flaw with the Pot Metal rotors. The long fin on the rotor extends over the steel hub that is cast into it, And the Pot Metal does not bond to the steel hub, leaving it cantilevered and unsupported. In the photos that Lightfoot posted you can see the end of the long fin is lifted away from the steel hub, on Dell's and mine you can see the end of the fin has broken off, in my photo the broken piece is sitting next to the rotor. On the brass rotor the long fin is part of the casting all the way to the end.

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