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A loose shower handle is usually due to the set screw. Lever-type shower handles use a set screw under the curve of the handle to secure it to the faucet. Over time, the threads on the set screw expand just enough to make the screw loose, but not enough to strip the threads. Many people use a liquid thread-locking compound to alleviate this issue. The problem with this is that it is often difficult to remove the screw when making repairs. The best way to keep a shower handle set screw tight is to use plumber's tape.
Place a towel or wash cloth over the tub drain to prevent the set screw from accidentally dropping down the drain.
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Remove the rubber or plastic plug covering the screw on the underside of the handle. This usually simply pulls out with your fingers, although you can use a pair of needle nose pliers if necessary. When pulling the plug away, watch for the set screw. If it has unthreaded, it will fall out of the hole when removing the plug.
Turn the set screw counterclockwise with an Allen wrench, if the screw remains in the hole. Carefully pull the screw out and inspect the threads as well as the tip of the screw. The tip of a set screw should have a taper with a slight edge. If the tip is rounded, replace the set screw with a new one.
Wrap plumber's tape around the threads of the set screw. Do not add more than two complete wraps.
Thread the set screw into the hole under the handle and tighten completely with the Allen wrench. The plumber's tape helps to lock the threads by providing extra width to the screw. Push the plug cover over the screw.