you can use an impact, shouldn't need too. make sure the flywheel puller is screwed as far as it can be into the flywheel. only use the proper puller. The swingarm is only torqued to like 68 ft lbs if i remember right,that i use an impact anyway. It should have a shim (washer) behind each bearing seal end cap, look to be sure. Swingarm should be snug sliding into the frame.