It should be as in the G4Polyhedra constructor: https://geant4-userdoc.web.cern.ch/UsersGuides/ForApplicationDeveloper/html/Detector/Geometry/geomSolids.html#constructed-solid-geometry-csg-solids G4Polyhedra(const G4String& pName, G4double phiStart, G4double phiTotal, G4int numSide, G4int numZPlanes, const G4double zPlane[], const G4double rInner[], const G4double rOuter[] ) zplane come at the end instead.