It looks like 1/4 in. pipe thread in the photo. Like jerry says, look in the hole. If there is no seat in it for the flare to seat on it is almost certain to be pipe threads. It will take brass fittings to adapt 5/16 inverted flare tube to fe male either 1/8" or 1/4" pipe threads. They will be recessed on one end & have male pipe threads on the other. They are available at parts stores or plumbing supply in the hardware store. If you haven,t stripped the threads when you tightened them down I think you will be OK.
