What kind of mag do you have?
I am familiar with the one on the 9u where the rotor will only fit one way.
Not sure what mag it is. The rotor is like a gear that can go on anywhere on the mag. Lost my book so don't know where to point it. I cant even remember which cyl is number one.
If you look closely at the teeth on both gears there should be a mark of some sort. I have seen two dots on the mag one and a clipped tooth on the rotor before.