From an old post of mine:
I wonder about the FSA/Shimano compatibility.
I just swapped a FSA crank from my 2009 Kona Unit with a SLX from another bike.
My intention was just to swap cranksets.
When I tried to install the FSA in the other bike with the SLX bottom bracket, it was tight going through the first bearing, had to use a dead blow hammer just a bit. However, when I went to seat the crankset fully in the bearings, I couldn't do it. I used the dead blow hammer but it wouldn't fully seat, at least 1/4" of shaft still out of the bearing - I was afraid to do it too much in case I wouldn't be able to get it back out.
I should have checked with my calipers but just swapped bottom brackets. Everything went together easy after I did that.
And yes, the bottom bracket had been faced and chased. Wonder if it was just tolerance stacking or if there is a slight difference.

And from someone else's post:
Shimano runs a -0/+.xxx tolerance and FSA is the reverse. If your lucky they will work together but more often than not they clash.