while i hope you luck with your endeavors I feel the need to point something out: like 2door said you may be having some other problem that this is a symptom of.
it would really suck to get your sprocket solid only to have your crank snap or something.
the idea of the key is to have a sacrificial part that breaks so more expensive/harder to replace parts can be saved. so it is a very bad idea to just "fix" the snapping key problem before diagnosing what is causing it.
chances are this is not an issue with your key or your sprocket, for example perhaps there's a warped set of links on your chain that is throwing the alignment off on something, or maybe a driveshaft problem.
basically what you should do at the least is make sure your crankshaft is balanced before you run it again.
this sort of problem could be a symptom of something much worse, your piston might not be mated to the crank properly, or you could have a crappy crankshaft that will do this over and over again until it gets replaced.
****, there could be a bad casting on your lower or something that's moving the gear/not Gripping a bearing correctly.
It sucks that the best advice for you is 'take the whole thing apart and go over it with a fine toothed comb' but it is what it is. again, good luck I hope you figure this out, and I hope it's a good story when you do.
awesome! good luck with the bike!
