First of all I operate an Exmark Metro 48". A few weeks back i hit a piece of metal hidden in the grass and it bent my blade. Getting the nut and bolt off was a pain in the A$$ and took two of us with box wrenches and helpers to get it off. So im cutting the other day and notice the same vibration. and the same problem getting the nut and bolt off. Things ive decided it can't be. -Blade is balenced -bolt is straight as an arrow -never seize was used I am kinda curious if any of you have any input because the only thing i can think of is that there is an excess build up of never seize with maybe some dirt or sand in it causing the nut to not turn off. or could the pulley be bent? i don't thinks so because my stripes are still very nice and there isn't any uneven ness in the grass. any suggestions would be much apreciated.