Repairs My blade won't stay tight? Also, should blade adapter have some play in it?

Picking up leaves I hit a baseball size rock which caused my blade to loosen a little. Now I can't get the blade to stay tight for more than 50 yards. I'm going to replace the blade but noticed the blade adapter has a small amount of play because the sprocket does not fit snugly to what I think is the crankshaft. I take it this happened as a result of the blade starting and stopping hundreds of times. Is this slight play in the adapter OK? If not, should I replace it?



Also, what can I do to get the blade to stay on tightly?



My mower is a Craftsman 917.388861. I have no problems with operation, beyond the blade.



BTW, I'm asking a second question on blades.