ring size on stock pistons

[ols0051]

I called summit and they say they are metric. But still waiting for them as they are on back order. They are not file fit. I wanted file fit rings because the plan is to use nitrous. But I also wasn't going to spray it until it was running perfectly. And as the summers seem to just fly bye it could be next year for that as slow as this thing is coming together. And next winter I was going to buy a new crank,rods,and pistons. What is the difference between file fit and non file fit? The non files just are made a spec right? or wrong? If I decide to open the gap up a bit I can right?