I put the forth gear drum in both my klx110s, personally its alot of work to take the whole motor apart to put the forth gear in. The cases need to be split to get to the trans so i highly suggest leaving it to a professional unless you have a nice %!¤nal of tools (flywheel puller, case spliter) an easier way to get more top end out of the bike would be to put a rev box in and a smaller rear sprocket.