A metric machine screw is 5 mm long, has a diameter of 2 mm and a pitch of 0.4 mm. This is about the smallest screw that is commonly available at a hardware store. The diameter of a threaded cylinder like a screw is measured at the largest cross-section (not at the groove where it is indented). So the thread of the screw forms a helix of constant pitch 0.4 mm and diameter 2 mm. What is the length of the thread (helix) that makes up 80% of the length of the screw?