The Altus FC-M311 3 ring chain set I installed on an old Giant MTB rubs on the bottom bracket. There are bent out bits of the small ring that are just slightly smaller than the diameter of the BB and act like a lathe to skim metal of the BB. No spacers or anything in the box it came in. Maybe I could file the lugs down? Any ideas welcome.
For the intended 50mm chainline, FC-M311 wants a 123mm spindle, which is almost the longest common length. I don't believe the issue you're having would be possible with that spindle length on either a 68 or 73mm shell.
If you're trying to get narrower chainline on purpose, this is probably not the crank for it, and yes you may need to hack it. If this is happening due to having installed the crank on whatever bottom bracket was already in there, then going to a 123 is likely to fix everything.
