[chevyrestoguy]

If you look on the boss where the allen bolt threads into, you will see a groove all the way around. The puller locks into this groove. Most pullers have a piece that looks like a socket that's split down the middle. This "socket" has a tang on the end that slips into the groove. You then thread a long rod into the center and it bottoms out on the area where the allen goes. The harder you tighten the rod, the more it pulls the pulley off of the pump.

Any major auto parts store (AutoZone, NAPA, O'Reilley's) will loan you the tool for free. You just have to leave them a credit card for collateral.