Fig 1R75Hover to traverse the gaugecrown · R 158 mm
Centre probe: the contact the dial readsShell section, cut as a templateDial limit mark: 75 mmArai state that every shell carries "a continuous curve radius of at least 75mm", so it glances off an impact instead of catching. A three-point gauge measures that directly: the circle through its three probe contacts is the shell's radius of curvature there. This section is a chain of tangent arcs of 96, 150, 158, 75 mm, so the gauge reads 75 mm at its tightest, on the brow, and more everywhere else. Representative geometry, not a scan of a production shell.