I have a three year old Stihl MS250 and have never had any problems with it until today. After I changed the chain, the forward most bar nut would not tighten like the rearmost would. I noticed that the bolt that is mounted in the housing was turning with the nut after a certain point. I had never noticed it as a problem before when adjusting the tension. Is it supposed to do this to keep you from tightening it too much? The bar only loosened once during three more hours of heavy cutting so the saw didn't let me down.