Small Engines (Lawn Mowers, etc.): Could wrong mower blade ruin the MTD 6 point star adaptor?, point star, star design

Question
I have a 1 year old MTD/ Troy Built 21 in. pushmower model 11A-546N711. I hit a rock and had to replace the MTD 6 point star adaptor, because the key on it sheared.  The store I bought the adpator from sold me a new blade as well but the blade had a rectangular slot to go over the 6 point star that was on the adaptor.  It was like a generic fit but not the star shaped design like the MTD/ Troy blades.  After one cutting, the two notches on the blade adapter sheared and the edges of the star design are all worn down and the blade just spins. The blade was on tight and I did not experience bad vibration due to the rock I hit.  Any help appreciated - I now need to get another adaptor as well as the right blade.  If the wrong blade did not cause the adaptor to fail I must have another problem. Thanks.

Answer
We sell a lot of the rectangle blades that you described.  We have not had any problems with the blades or the adapters.

How tight did you tighten the blade bolt?  Is the crankshaft bent?  You can check the crank by removing the spark plug and having some crank the engine while you observe the blade/crank.  Normally one of us will stand on the mower handle and shut-off bail and pull the string while the other watches the crank.
If the crank appears to wobble then it is bent.  This can cause big problems.

Usually when you hit an object the flywheel key will shear before the blade adapter is damaged.  Have you checked the flywheel key?
Where did you buy the blade adapter and blade?
Let me know.
Eric