Hi Gus, I must enquire with dealer in Victoria about putting 18T on my C5.
One of the major meeting places is Alices' Restaurant ( yes, the one in the song ! ) at a crossroad in Woodside
Because the C5 has smaller tyres it has a larger gear (18T). Conversely the G5 has 17. You can put an 18T on a G5 but cannot go any higher on a C5 because of clearance issues .
Im running a 19T front sprocket on my C5.. No clearance issue whatsoever. Infact one could run a 20T provided you dont have the kick start lever.
Is that an RE part? Did you have to pull the primary to do it?
I agree with Br. Kevin, there is not much room to go with a bigger sprocket on C5 which we can say is safe enough. Hitchcock says if you don't have a kick shaft then its fine to go with 19, 20. I think otherwise as there is hardly 3-4mm gap between the chain and engine chamber and going bigger is surely not safe. See for yourself the gap with 18T sprocket.
I am sure you can run 20T too but I am not too comfortable with leaving too less space for any mishaps. If you keep the rear sprocket and chain in good health then you are safe as long as you keep things under check.Once I was told a story of a guy who lost his life on TBTS when for some reason the chain got stuck in front resulting in breaking and jamming up the engine causing serious accident. The black stopper on our swing arm is a result of that serious event to counter the problem of chain bouncing when the chain is too loose.I am just a bit too fussy about things like that but if you keep things in check then you should be safe.