Warning to those w/teh Sunstar sprocket. It doesn't like to stay on. The nut on the engine shaft will come loose after 3-4hrs ride time no matter how tight you get it on. The sprocket needs to be grinded down or something. I'm not sure how it does it but it does. I even switched to a different nut and the same thing happened in the middle of a race on Sat. Was able to get it tight enough w/my hand to finish the lap and do a quick tightening and still win the class, but not by much.
Are you bending the lock washer? It should keep the nut from spinning loose.