The foot long metal rod that J. Hidalgo mentioned is the way to go, but since you're into it, a piece of nail about the size Sixto described will do in a pinch.

Once the hole in the back of the pulley is lined up with the ridge, lay the nail in it and push into the pulley hole.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"