J. W. Winco, Inc. GN 743.8

The conical thread of GN 743.8 fluid level sight glasses makes a metallic seal possible. When the male thread is tightened, it wedges itself within the appropriate female thread. At the reference level "a," the male thread has the same thread diameter as the female thread, so that it can be fastened in by hand. The secure tightening by means of a tool increases the length of engagement and seals the screw connection. Additionally a sealant (fibrous or teflon band) is usually used. The thread is "roughened," so that the sealant does not shift during fastening. Fluid level sight glasses GN 743.8 can be used on pressurized oil tanks.