A rigid spacer for the PTFE/TFM with a viewing hole where one can check that the bowden tube is properly seated. This spacer can also replace the sometimes problematic spring on the UM2-machines. I made the spacer 9.6 mm thick, compared to 9.4 mm for the rigid metal spacer on the UM2+, to compensate for bed-leveling inaccuracies. This means that in normal cases there should be a small gap between the heat break and the block when tightened (see picture). I recommend to use ABS, CPE or other materials that can take a bit more temperature than PLA for this print. 100% infill works best.