I doubt they are through holes. So best you make sure you have the correct length bolt.
Take the drain off and measure the depth of the hole. If you don't have a set of verniers, then you could just use a tooth pic - poke into hole and use your nail or a pen against the side of the tooth pic and the hole to mark the depth.
Then make sure your bolt is no longer than that + the thickness of the flange.
Ok re-reading your post, you say the bolt is tight but it won't do up anymore, but then you mention using a helicoil.
Is the thread busted in the sump flange (cross threaded, stripped etc) or is the bolt too long? A helicoil isn't going to help if the bolt is just too long for the hole....