A slip nut is used to connect a tailpiece to a valve body when the nut inside diameter is too small to fit over the outer diameter body of the tailpiece. Examples are shown on page 75 of the List Price Catalog March 2016 and in the images below.
These fittings are used on many products including the 521 Series thermostatic mixing valves and the Z2, Z3 zone valves. The slip nut can be used wherever a standard union nut is used, with the exception of SolarFlex stainless steel flexible tubing. The clip used with SolarFlex requires the full flange of the standard union nut, to assure a watertight connection. The slip nut fits over the tailpiece from the flange end of the fitting, not the pipe connection end, so the nut does not have to be installed on the piping before the tailpiece is connected to the piping. Because of it's "universal" appeal, we'll be using slip nuts with more of our tailpiece fittings as new sizes are developed.
To connect a slip nut to a tailpiece, angle the tailpiece flange upward, about 45 degrees, and slip it into the nut flange cutouts, with the larger section of the nut flange at the bottom (the cutouts are at 10:00 and 2:00) and the smaller section of the nut flange at the top. Image A, the other images, and the quick video below are worth a thousand words...
