I think you over tightened them and if you did you're taking a chance on cracking them. When you tighten the bolts you only have to go about a quarter turn past tight and the bottom of the bolt should be flush with the bottom of the blade. When you took your old blades on did the bolts have washers in them? If you're worried about the blades coming loose put a drop or two of the removable (blue) Lok-Tight on the threads of the bolt. I do that with all my blades and never had a problem with them coming loose.