Transmission. If you can, use a 5 speed bottom end. ( fits ok in your 4-speed frame... if you have a 4-speed ). I suggest the CB500T transmission. It has closer ratios and all the turning gears are fully bushed, prevented the "dropped into two gears at once, while cranked over" syndrome, which can happen to worn CB/CL 450 transmissions. It goes right into the 5-speed case, with no modifications. Back