When I did mine, slide the impeller down the shaft till you get to the flat spot on the shaft, hold the pin in the flat area as you bring the impeller down, it goes pretty good that way, the pin will be in the middle of the flat area. I dont know if that makes sense to you, if you have the impeller in position now, you will have to bring it up some so you can put the pin on the flat, hope that helps.