Where exactly an how has the thermal paste to be applied to the heat-break ? on the screw part only or also higher on the plain end?
It goes on the long part of heat break that is screwed into the heat sink. It shows a picture if you scroll down in the link below. http://wiki.e3d-online.com/wiki/E3D-v6_Assembly#HotSide
Little OT here maybe... does BB with PT100 have something like E3D-v6.5 hot end? This normal v6 with the thermistor on the screw with washer on the side confused me a little with assembling.
It's a regular V6 just with an alternative heater block to fit the PT100. You can buy the block and the PT100 as an upgrade for a 'normal' V6.