marble run transportation snail with 7 twist for fischertechnik and other. Designed for 13mm marbles. The inner axis in the figure 4.5 mm in diameter. My Prusa I3 makes it just a 4 mm diameter. Compatible with the 4mm axis of fischertechnik. Pleas use sufficient Brim or Raft for secure footing. I have reduced due to the acceleration forces, the print speed to 20%. Z-Axis: 0.2 mm PLA 220 centigrade Speed 50 to 10 mm/sec Heat bed 55 centigrade Adhesion promoter: Hair spray