okay well can anyone tell me about cranks i been trying to look up about size and stuff and seem to find only that the length matters? do I need to worry about if it is going to fit together right on the bike? as far too narrow, too wide or just not be the right size? I'm still reading up on it but all of the info i read about seems to only discuss the length, which i think i will go with a 180 mm but yeah. Is that All i am looking at, would it be safe to just go on e-bay and get 180mm single speed mountain bike cranks?