Patent ID: 7293628

Claim:
A shell main body for muffler, wherein the shell main body is made of one rolled flat plate and a joint portion of the plate is welded, the shell main body comprising: an inner shell formed by a part of the plate to have a cylindrical shape with a cross section that is not a perfect circle but is defined by small arc portions alternating with large arc portions; and an outer shell formed by another part of the plate that continues from a portion of the inner shell to wrap an outer surface of the inner shell and having a cylindrical shape with a cross section that is not a perfect circle but is defined by small arc portions alternating with large arc portions, wherein the small arc portions of the inner shell are in close contact with the small arc portions of the outer shell; wherein the large arc portions of the inner and outer shells form a gap therebetween having an upper gap and a lower gap at an upper side and a lower side thereof, respectively; wherein at least one of the inner shell and the outer shell is provided with at least one radially projecting bead that extends continuously in a circumferential direction along the small arc portions of the at least one of the inner and outer shells to define a passage inside of the radially projecting bead so that the passage can allow the upper gap and the lower gap to communicate with each other; and wherein the outer shell is provided on an upper surface side thereof with a communicating path allowing the upper gap and an outside of the outer shell to communicate with each other.