Cast iron pulley or Ali ?

[rusty nuts]

When i fit one of those Ali crankshaft pulleys after a time the slot open's up and then the wheel has movement. Question is this, do i have to use a Ali pulley, the car does have a Ali head on it. It did have a cast iron one on at first and even that slot had opened and a crack appeared.

I'm thinking a cast iron pulley would take a bit more before coming loose, i often wondered whether the alternator was a tiny bit out causing the slot to open.