I have a honda XR 125 that is leaking oil slightly. I did change the gear selector shaft about 2 years ago and didnt notice a leak then. I have a feeling I was meant to replace the seal around the selector shaft. IS this a common problem? Does anyhone know if I have to remove the clutch and selector shaft in order to change the seal or can it come out the other side?

Much appreciated for your help

 

pretty common to change that o ring whenever you open up that crankcase