Aluminum- light; but my harmonic balancer is aluminum and the alum. gets eaten away. Now there is oil comming out of it. Because the balancer spins, the oil flies everywhere. It sucks. Now i have to either put the stock one back on or buy a special sleeve to go over the aluminum. Either way, it kind of sucks. I'd go with whatever. Nobody else i know of with the alum. ha had the same problem as me.
Never had a problem with the aluminum pulleys. Some pretty hard driving.