O.D. stands for outside diameter, the distance around the outside of the copper pipe. The outside diameter is always larger than the inside diameter of the pipe because it takes into account the thickness of the copper. Confusingly, the outside diameter measurement has little to do with the measurement given in the name of the pipe.
Nominal Pipe Size and Outside Diameter
Hardware stores stock copper pipes labeled 1/4 inch, 3/8 inch, 1/2 inch and so forth, but this measurement does not refer to the inside or outside diameter. Confused? Back when pipes were all manufactured with a 1/16-inch wall, the outside diameter was always 1/8-inch greater than the inside measurement. Nowadays, however, manufacturers are able to vary the thickness of the walls to accommodate different pressures of liquids. The prevariation measurements still remain the standard way of sizing copper pipe, meaning that a 1/4-inch pipe, for example, will have an outside diameter of 3/8 inch and an inside diameter of 0.305 inch and up, depending on the thickness of the walls.
Wall Thickness Grades
The thickness of the copper falls into four common groups – K, L, M and DWV – with K the thickest and DWV the thinnest. ACR and Medical Gas grades are also available. ACR comes in flexible and hard forms. Medical Gas is, not surprisingly, used to carry medical gas. The thickness grade is always marked on the pipe using color coded labels. K is green, L is blue, M is red and DWV is yellow. Determine the copper thickness required for the task to avoid cracks or failures of the material.
Take into account the measurements of the existing pipework if you are working to repair a cracked or damaged section of pipe. In some cases, a pipe with a larger O.D. can be slipped over a damaged section and soldered into place. Always ask for help if you need it.