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This is probably a stupid question. I just checked the gap of my oil rail ring and it's currently at 0.012". The specification recommends 0.015" at the minimum.
Do I just file the oil rail the same way as filing the top and 2nd ring to achieve at least 0.015"?
Yes, exactly the same technique. In my own experience it's uncommon that the oil ring rails need adjustment but not unheard of. Just file them to suit and you're good to go!