A SLIP-ON SPOUTS slide over 1/2-in. copper pipe and fasten with a setscrew. This "universal" version also has threads inside, so it can screw onto threaded pipe.
SCREW-ON SPOUTS have threads deep inside. They can connect to a copper threaded fitting or to steel pipe.
SCREW-ON SPOUTS may have threads at the back end. Most come with a bushing so they fit either 1/2-in. or 3/4-in. pipe.
1. Unscrew the old nipple with a pipe wrench. If the nipple is too short to grab with a wrench, use an “internal” pipe wrench ($10).
2. Wrap both ends of the new nipple with thread tape and screw it into the fitting inside the wall. Seal around it with silicone caulk and screw on the new spout.
clipped from: www.rd.com 
