Ive got an 87 xr600 that Im putting an 18 inch rim instead of the 17 (48tooth)currently on it. Does any one know whats the biggest sprocket I can put on it without hitting the chain guard. The 18 rim is supposedly off of an 89 and stock for that was 50. thanks. will I need to add more links to the current chain to facilitate the 50 tooth? if i go bigger I expect to add chain but am afraid of hitting the chain guard.