I was doing some searching and found a chart that says the 21/40 gearing is no good. http://www.hardcoresledder.com/SNOWMOBILESPROCKET.htm why can't that combo be used? Also, how do you figure out the pitch of the chain? I put the gears and chain in and everything seems to work fine, and the chain I have is a good length. maybe I'm missing something? any help is appreciated.