Install the puller as far as it will go WITHOUT the plunger (center piece). Now install the plunger and tighten it down as tight as you can by hand. Take a hammer and strike the plunger a sew tines. Tighten plunger and strike with hammer again...repeat tightening the plunger/strike with hammer. Be careful that the puller is seated as far as you can...they aren't good tool steel and can strip the tool or the sprocket threads. You may need to jam the two gears with a soft metal or a thick rag.