ridge 8 in turn having an essentially equal sectional shape along its extension the thread groove 9 has a thread bottom 12 exhibiting a plurality of surface portions having part circular shape which pass over into each other namely a first surface portion y 1 having a radius ra closest to a first thread flank 10 comprising pressure flank a second surface portion y 2 having a radius rc closest to a second thread flank 11 and an intermediate here central surface portion yc having a radius rb the surface portions pass evenly that is tangentially over into each other this is preferred even if it can exist linear passages and also which is not recommended uneven passages having angular steps between the surface portions the treaded flanks form the same angle to a symmetry axis of the thread normally this angle is 35 u00b0 in certain cases other angles can exist for example 45 u00b0 the radius rb of the central surface portion yc is greater than both radiuses closest to the two thread flanks 10 and 11 suitably there is a ratio between ra respectively rc and rb of about 0 05 0 70 preferably of about 0 30 0 60 and most.