Lets try a bit of logic .If it was a crack it would have probably opened up to the point that you would not get the top nut to thread on. If the top nut still threads on easy then it is probably intentional.Is a stem loose in the steerer when it is put in or does it just slide in nicely. If it is a very close fit then it is unlikely to be a crack.Also when you have the top nut on it really cant go anywhere anyway. Is there some sort of internal bevel in the top nut that clamps the steerer top in on to the stem when it is tightened? course could be wrong on all of this so tis only a thought.You would think that a crack in the threaded section would tend to follow the threads a bit as against straight down like this is. Sorry but if no one knows for sure you really only can use your intuition to know what to do.
