[asid61]

You would need a 1/2" hex broach or turn down the 1/2" hex to the bore of the pulleys.
I would recommend using #25 chain and sprockets instead, or even #35, as it is readily available at Vex and Andymark, is much more forgiving in terms of strength and precision, and comes in 1/2" hex bore sizes.

EDIT: Those are MXL pulleys, which are not commonly used due to their lower working load. GT2 or HTD pulleys are generally better for our purposes.