Choosing the wrong drain fitting size can cost you a return trip to the hardware store. Kitchen sink compression fittings are available in two standard sizes, and manufacturers indicate a fitting's size according to the diameter of the pipes that the fitting connects. If you plan to replace your kitchen sink's drain assembly, it's wise to remove the pipes and fitting before you purchase replacement parts. You can take the removed assembly with you to the hardware store to ensure that you buy matching components.
Common Fitting Sizes
The most common kitchen sink drain fitting sizes are 1 1/2 and 1 1/4 inch. These measurements refer to the diameter of the pipe that the fittings connect. Notably, kitchen sink drain assembly pipes and fittings are not directly compatible with inwall drain, waste and vent pipes. To connect drain assembly fittings to ABS or cast-iron drain lines, plumbers install size-increasing fittings called adaptors.
Drain Sink Fitting Types
A basic kitchen sink drain consists of a wall drain extension pipe, a trap and a sink basin extension pipe. Wall drain extension pipes connect to ABS or cast-iron wall drain stubs via an adaptor fitting. The wall drain extension pipe connects to a drain trap with a compression nut fitting. And the basin extension connects to both the trap and sink strainer with a compression nut fitting.
Standard kitchen drain assembly components connect via compression nuts and plastic ring washers. For example, a plastic ring washer and compression nut slide over the smooth end of a basin extension pipe. The smooth end of the basin extension pipe slips within the opening of a drain trap. To connect the pieces, a plumber slides the nut and washer down the shank of extension pipe and joins the nut with threads moulded around the circumference of the trap's opening. As the nut joins the trap, it compresses to the plastic ring washer to create a watertight seal.
How to Determine a Fitting's Size
To determine a fitting's size, measure the diameter of the pipe connected to the fitting. If you attempt to measure the pipe's diameter with a tape measure, centre the tape across the pipe's opening to ensure an accurate reading. Alternatively, calipers offer the most accurate method of measuring a pipe's diameter. Fitting manufacturers always indicate fitting size before an item's description --- for example, 1 1/2-inch coupling or 1 1/4-inch adaptor.