1/4" Copper Union

Supply House's unions worked fine for me and the price was right. I see a lot of other amateur plumbers complaining about how hard it is to get copper unions to seal, so I'll share my union theory for what it's worth: The watertight seal in a union comes from the mating surfaces deforming enough to make up for slight imperfections in the material. For that to happen they need to be manufactured correctly and you need to squeeze them together pretty tight on installation. Look the mating surfaces over before installing and make sure they're clean and there aren't any obvious irregularities. You can lap the joint with automotive valve lapping compound if you want, but make sure you wash all the abrasive away afterward. Pipe dope or tape are neither required nor desirable on a copper union. A brass nut on copper threads is a recipe for seizing, so you might be able to save yourself some grief by putting a little dab of anti-seize compound on the threads and on the contact area where the nut rotates over the flange of the male side. Also try to locate unions where you have good wrench-leverage to the extent that this is possible.

We recently purchased a number of these unions in both the 1 1/4" and 1" sizes, and more than half of them leaked regardless of how tight we made them. They had to be replaced in each case and I am totally dissatisfied with this product.