mercury 1984 water pump

[steplift20]

First of all happy thanks giving to all my friends out there
now this is my problem
I have a 1984 mercury out board that i am changing the water pump on. I put the motor in forward took off the lower unit and now i cant get it to go on all the way, it stops about 1 inch away and i cant see what is making it not go all the way up
any help would be great thanks again