Patent ID: 7735473

Claim:
A connection head structure of a high pressure fuel injection tube comprising, on a connection end portion of a thick-walled thin-diameter steel tube, a spherical seat face, an annular flange formed at an interval from the seat face in a tube axial direction, and a conical surface having a substantially spherical face which tapers toward a tip and continuously extends from the seat face to the annular flange or to a proximity of the annular flange, and a fastening nut is assembled to engage directly or indirectly with a back face of the annular flange, wherein when the thick-walled thin-diameter steel tube has wall thickness t/outer diameter D<0.3, a tube axial direction distance L 1 from a connection head end to the back face of the annular flange is from 0.38 D to 0.7 D, a spherical body radius R of the seat face is from 0.45 D to 0.65 D, and an outer diameter D 1 of the annular flange is from 1.2 D to 1.4 D, wherein an inner circumferential face of the connection head comprises a two-stage tapered face having a cross sectional outline in a tube axial direction of the steel tube that broadens toward an aperture of the tube.