And if the start of the thread is dressed to form a fairly short steep ramp it aids in starting the thread really nicely. It involves turning away the last bit of threading for a distance of roughly the diameter of the screw so it slips into the nut easily and acts as a pilot. In machining circles there's a trick for finishing the end of the thread called a 'higby end'. The ring can be either a real O ring or just a short length of vinyl tube that fits tight around the thread. Just counter drill CAREFULLY from below or into the deck of the model for the O ring. It does not look like there's a lot of room for an O ring without doing some butcher work.