For sportsbikes i can tell you that the front sprocket is just under a cover to keep some dirt (and your foot) out of it. Follow your chain and it'll take you there, usually just under a cover held on by a few screws. I can't imagine cruisers, old or not, are any different. if your sprocket was in a closed oil system, your chain would be slinging all that oil out. i think you are confusing sprockets and transmission gears. hopefully youll never have to see the latter.