Filler plug for hydraulic device

A metal filler plug for a hydraulic device and for use with a tool, the hydraulic device defining a through hole and an internal thread portion formed therein, the hydraulic device also defining a funnel-shaped inclined inner peripheral surface at one end of the internal thread portion, the plug comprising:

• an external thread portion that defines an upper end portion;
• a head portion, having a larger diameter than the external thread portion, formed on the upper end portion of the external thread portion, the head portion defining an upper end portion and an engaging recess formed on an upper end surface thereof;
• a circular sealing portion formed at a lower edge portion of the head portion strongly contacts the funnel-shaped inclined inner peripheral surface by virtue of the tool engaging with the engaging recess of the head portion and the external thread portion thereby being tightened with respect to the internal thread portion, the sealing portion defining an upper side, the sealing portion extends outwardly in a direction that is away from the external thread portion;
• an upper peripheral surface portion located at the upper side of the sealing portion, the upper peripheral surface portion being formed as a tapered portion configured to increase in diameter in a direction from the sealing portion toward the upper end portion of the head portion;
• and
• a circular constricted portion having a circular-arc cross section formed between the sealing portion and the upper end portion of the external thread portion so as to couple the sealing portion with the upper end portion of the external thread portion;
• wherein a dimensionless value of a head portion height from the upper end surface of the head portion to the sealing portion comprises a dimensionless value being obtained by dividing the head portion height by a nominal designation value of the external thread portion;
• and
• wherein the sealing portion strongly contacts the inclined inner peripheral surface to perform plastic deformation.