Kevin Mckenzie

Hello gents, Can any one help me with my gearbox problem. I cannot get any higher than 4th gear during high revs but when I am in low revs I can.The gearbox does not jump out of gear at any time. I removed the right side cover and noticed that the shift lever with the gear selector arm is not making contact with dowels for changing the gear and that there is wear on the rivet that holds the C" clip on the end. I also noticed that the rivet has touched the back of the clutch basket due to side travel of the gear shit shaft. you can push the gearshaft back in an the gears change okay. what is causing the gear shift shaft to move as the dowel on the casing is there to stop it moving to much . Thanks Kev