don't hammer them, put them on the hole, flip the wheel upside down with the bearing still on the hole, and push as hard as you can on a flat surface that's how I got the bearings in my new gms
best way ever. put the bearing on the gounrd- floor, on ther opposite side that you are going to step on the wheel. step on the wheel witht he side that the beaering isnt on. Put spacer in and do the same for the other side.