If your bathtub spout doesn't meet the back wall of the tub, chances are you chose the wrong type of spout, but even if you didn't, the fix is fairly easy. The most common spout has screw threads for the pipe nipple that extends from the wall near the front, where the water comes out.
But on some spouts, these threads are near the back, and the latter type won't fit on a nipple intended for the former. If your spout has threads near the front, or it's a slip-on spout, and it won't seat against the wall, the nipple is too long and needs either to be cut or replaced.
Three Types of Bathtub Spouts
A bathtub spout is hollow inside except for a connector welded to the sides. This connector usually has 1/2-inch female pipe threads, which means the pipe sticking out of the wall, called a nipple, has to have 1/2-inch male threads. Some spout connectors are slip-on and connect to a smooth 1/2-inch copper pipe, and they have a set screw that you tighten with a Phillips screwdriver or an Allen wrench to hold the spout in place.
The most common spout has threads near the front of the spout, and if the spout has a diverter, it's also near the spout outlet. Less common are spouts with connectors set near the back, and you can identify one of these because the diverter is also located near the back.
The third type of spout — the slip-on — has an internal water passage into which the copper nipple fits, and if it has a diverter, it's near the front. You can easily identify a slip-on spout by looking for the set screw on the underside near the wall, which a screw-on spout doesn't have.
Threaded Nipple Is Too Long
The galvanized steel or brass nipple for a spout with a threaded connector near the front should be about 2 to 3 inches long, and if it's slightly too long, the spout won't sit flush. You can try tightening the spout an extra turn or two, but besides being difficult to do, it's risky. You could damage the threads of the spout or the nipple.
A better solution is to unscrew the nipple, which is connected to an elbow just behind the wall, and replace it with a shorter one. According to EMI Supply, pipe nipple lengths vary by 1/2 inch, so if you have a 3-inch nipple that's too long, replace it with a 2 1/2-inch one. To do this, unscrew the nipple with a nipple extractor and screw in the new one. Don't forget to wrap plumbing tape around the nipple threads before screwing it in.
Some spouts come with plastic extension nipples, as described by Danco. If you replace a conventional spout with one of these, you might also need to replace your existing 2- or 3-inch nipple with a 1-inch one to get the spout to fit flush against the wall.
Smooth Nipple Is Too Long
A slip-on spout connects to a length of unthreaded copper pipe, and this is usually soldered to the elbow behind the wall, so it isn't easy to replace. If it's too long, though, you don't have to replace it; all you have to do is cut it. Use a tubing cutter rather than a hacksaw, because it leaves a smooth edge that will slip readily inside the slip connector.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.