On a very rough-and-ready basis we might define an eccentric as a man who is a law unto himself, and a crank as one who, having determined what the law is, insists on laying it down to others. An eccentric puts ice cream on steak simply because he likes it; should a crank do so, he would endow the act with moral grandeur and straightaway denounce as sinners (or reactionaries) all who failed to follow suit. Cranks, at their most familiar, are a sort of peevish prophets, and it's not enough that they should be in the right; others must also be in the wrong.
