Bathtub drain parts are divided into two categories, the parts that are visible and the parts that are installed behind the wall and under the floor. Understanding the various parts of a tub drain and their functions is important to understanding how to repair the drain when it starts to malfunction. Although the appearance of the drain components may vary slightly based on manufacturer, the basic drain design is the same for all tubs.
Look inside the wall of the tub, just underneath the faucet and locate the round metal circle. This is the tub's overflow valve cover, or overflow plate, which allows excess water to drain out instead of spilling over the tub wall. The overflow cover is attached to the tub using two screws. In some tubs, the overflow valve also contains a trip lever, which is used to stop up the water in the tub when bathing.
Look in the floor of the tub under the faucet and locate the round circular hole. This is the actual drain hole through which water drains out. The drain hole may be open, or may contain a metal strainer on top of it. Inside the visible portion of the drain is a threaded drain gasket, which allows a drain stopper to be installed in some tubs.
Locate the drain stopper, which is positioned inside the drain hole in some tubs. The drain stopper is a round metal device with a rubber gasket bottom that is pushed down to hold the water inside the tub. If the tub does not have a stopper installed, then it has a trip lever attached to the overflow valve cover.
Follow the back of the trip lever down to locate a 1 to 2 inch piece of threaded metal called a linkage. The function of this metal piece is to link the trip lever to the rods in the bottom section of the drain. At the bottom of the linkage is a plastic coupler sandwiched by two lock nuts that rotate around the linkage. These nuts allow a plumber to shorten or lengthen the rod to fit various tubs.
Look at the bottom of the linkage and locate the rod that serves to link the upper portion of the drain system, mainly the link lever and linkage piece, to the stopper that is located at the bottom of the drain. The rod is approximately 12 to 18 inches long.
Examine the bottom of the rod and look for a large cylinder shaped device that is constructed of hard plastic or metal. This is the stopper and is the part of the drain that actually plugs it up and prevents water from seeping out.