Don't know why this is happening but maybe you can shed some light....Have a new Super Z w/ XR-7 deck... I have change the blades several times with no problem. I go to change the set of blades (4th set) and I can not break the blade bolts loose... on all three blades. They are so tight I have to put a pipe on my wrench and hold the blade as best I can to break them loose. Thought it was just a quirk but the next time I tried 2 out of 3 were nearly unbreakable......Don't understand it. My other mowers never had this happen as this mower had not until now..Am I doing something incorrectly or do I need some special tool. Can you help?