I upgraded the BB/Crank set about six months ago on my Trek 6500 MTB. I wanted a higher gear and the XT came with a large ring with 5 more teeth that the stock unit. I did not have the XT tool and I had never done this so I had the LBS install a unit I purchased on-line. Well, I was not real happy with the unit and was concerned that it was making rubbing noises in some gears and miss shifts occationally. I took it back and had them re check and no improvement.
I was thinking about putting the old set back on. I decided Id take a look myself. I got the instructions out and read them very carefully. I noted after a couple of reads thru that it appeared that the LBS had placed the wrong spacer according to the instructions on the chainset side of the BB. The set came with a small bag of spacers and there were three sizes. My configuration called for the largest spacer and the medium size had been installed. There is only 0.7mm differance and I did not think that it would make any differance. But I took everything apart and used the correct spacer putting things back together.
I was very surprized but this small change made all the differance. The rubbing and scraping sounds have disappeared and the shifs are slick now.