“I need an O-ring, and if you have one that’s just a little bit bigger, that would be great.”  I hear this a lot from customers who come in with O-ring Boss fittings.  They’re here because they’ve tried skinnier O-rings, thicker O-rings, even O-rings out of a kit they had in the back of their garage for years; but no matter what, the fitting will not stop leaking. What they haven’t tried, however, is the RIGHT O-ring.  O-ring Boss fittings, or SAE straight thread, are designed to sandwich an O-ring between the angular sealing surface of the female port and the shoulder or back up washer of the male end.  Sealing is made possible by compressing the O-ring.  The O-ring is a specific size, and has to be replaced with an O-ring Boss O-ring.  So, the next time you have a leak on a Boss fitting that just won’t quit, remember, Boss is Boss.