The old ones were probably put in and then the edges peened over to hold them. You'll probably need to file/grind those peened spots a bit for the new plugs to fit in. They should go in to the bottom of the groove with a little hand pressure or maybe a tap or two around the edges. Then flatten them with the flat end of the ball peen hammer (not the round end). That should expand them to fit the groove tightly. Then peen a couple spots around the edges so they don't get forced out when you grease the bushings.
