Powerhouse Products sells an economical ring grinder that does a much better job than trying to gap your rings with a hand file. Simply lay the ring on top of the apparatus, butt the ends against the locator pins, and squeeze the ring so that both ends make contact with the grinding wheel. Spin the wheel so that it is pushing the ring down and into the locator , and grind away. Be careful not to cut away too much at a time; you can always cut more, but you can't add metal back to a ring. Many engine builders say that if your target end gap is 0.018 inch, you can get away with 0.004 to 0.005 inch of extra gap without harming performance. Don't, however, leave a gap too tight.