Open stem plugs use the same straight-sided rubber, large wing-nuts and corrosion free materials as closed plugs except they have an opening that passes through the center of the stem. This opening allows introduction of air or water pressure into the system you are testing. National pipe threaded tops on the open stem plugs make these versatile tools easy to set up for pressure testing, clearing blocked lines, or blowing out lines for winterization. The plug top is a combination 1/8" pipe thread and 3/8" hose barb.