To avoid lowering of jointing strength at the joint of a hollow cylindrical member and a blocking member.
A piston material for manufacturing a hollow piston is equipped with a piston body member 162 integrally provided with a bottomed cylindrical part 170 having a bottom wall and a hollow cylindrical part 174 and the engagement part, and a cap 164 to block up the opening of the bottomed cylindrical part 170. The cap 164 forms a bottomed cylindrical shape provided with a bottom wall 192, a cylindrical large diameter part 194 axially extending from the peripheral part of the bottom wall 192, and a cylindrical small diameter part 198 axially extending from the inner peripheral part of the end surface 196 of the large diameter part 194. A ring-shaped clearance groove 220 is formed in an area adjacent to the end surface 196 of the peripheral surface 204 of the small diameter part 198. The peripheral surface 204 of the small diameter part 198 of the cap 164 is made to engage with the inner peripheral surface 178 of the hollow cylindrical part 174 of the bottomed cylindrical part 170 up to the depth where the end surface 230 on the opening side abuts on the end surface 196, and thereby, the end surfaces 196, 230 are mutually welded as welding surfaces. The end surface 196 can be welded to the end surface 230 without producing a gap between them thanks to this clearance groove 220.
TAKAMATSU MASATO
HOSHIDA TAKAHIRO
ENOSHIMA FUMINOBU
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