What causes the threads to get stripped. Can it be caused by the person who rotated the tires and over tightend them or something like that. If the wheel is the exact same offset as the stock wheel than it can't be to thick for the spindle, right? Anybody got a clue on the causes?

 

Do you mean the studs that the lug nuts screw down upon? If one of them is stripped, it could easily be due to an overzealous installer, or dirty threads on the stud. After a wheel change, it is important that your lug nuts are not overtorqued, 85 lb-ft should do it. I have personally seen the damage that overtorqued lug nuts can do on my friends S2000. It took an air chisel to get the fastener apart!

There is no reason why the spindle nut (center axle nut) should be involved in a wheel change.