I just picked up a 550 XP yesterday and couldn't wait to get it home and run it despite the monsoon conditions we are having here. I specifically remember telling the dealer I wanted a .050" gauge 18" bar and he said that's what it came with so I didn't even look. I headed out of there with my saw, complementary new hat and a 6-pack of oil (which got me another year on the warranty) and got home to find it is a .058" gauge bar/chain. Sooooooo.... I figured I would remove the B/C and take it back to get what I want. I still wanted to run the saw so I took a used b/c from my Husqvarna 450 and put it on with a Stihl full chisel chain. It bolted on perfectly and I verified the oil hole lined up, I tensioned the chain properly and went to spin the chain on the bar and it felt tight and herky jerky. Not smooth like I would expect. So I figured it was just dry and it would free up a little when running the saw. Got outside, set the choke and fired it up. Hit the trigger and it was bogging. Figured it was cold so I gave it a second and it seemed to accelerate better so I went to put it in some wood. Started cutting and it was evident something was wrong. Shut it off and went inside, pulled it apart and I could see where the chain was rubbing on the steel guide plate above and below the oil hole on the saw. OOOOppps. I effed up my new saw already!!!! It will be alright, it didn't wear through but it would have if I kept running it! I held up the bar that came with the saw which is part # 5089121-72 to the used bar part # 5089261-72 and the heel of the bar where it mounts to the saw is much narrower which allowed the chain to rub on the back plate of the saw. I know the bar has some wear but I can't believe it has that much. I looked up the Husqvarna B/C fit up chart to see what the differences between the 2 bars are and of course they don't have the chart updated to include the 550 XP yet. They also have two different fitments for the 450. One is HLN-250-72 and the other is HL258-72. The HLN is a narrow sprocket nose .050" gauge and the HL-258 is standard nose .058" gauge. The used bar I put on must be a narrow sprocket nose/narrow heel bar and it doesn't work!! So after all this I guess my question is does Husqvarna make a .050 gauge bar that fits this saw or do I have to go buy all new .058 gauge chains for this????