What engine are you putting it on? The adapter goes inside the hole on the back plate and should be on the shaft with enough clearance to fit on, but not have movement. You can use brass tube as another spacer to go inside the insert, but you shouldn't have to do that. The insert should fit the shaft correctly to center tha back plate. If not, you need to get one that will. Anyone that has been flying a long time in your club should have them. Ask around and see if you can find one the correct size, otherwise you will have to make one from brass tube, which is doable, but I really don't like to do if I don't have to. I have done it a few times though.