Oil level sight glasses with conical thread

The conical thread of the oil level sight glasses GN 743.8 makes a metallic seal possible. When tightening the conical male thread R it blocks itself with the appropriate cylindrical female thread Rp.
With the datum plane a the R-male thread has the same thread diameter as the Rp-female thread, so that it can be screwed in by hand. The strong tightening by means of a tool increases the length of engagement and seals the screw connection. Additionally a sealant (hemp or fluoropolymer band) is usually used. The R-thread is „roughened“, so that the sealant does not shift when screwing in.