If you don't want to invest in a piston ring file a womans nail file works better than a standard flat file. You can get a package of them for a couple of bucks in the cosmetic section of the local pharmacy or department store. The nail file is an abrasive grit (i.e. saad paper) on a semi rigid fiber board with coarse on one side an fine on the other.
You can also use an inverted piston to square the ring in the bore for checking the gap.