Patent ID: 7364013

Claim:
A shell main body for a muffler, the shell main body comprising: an inner shell formed in a cylindrical shape with a cross section that is noncircular and defined by left and right small arc portions alternating with lower and upper large arc portions; an outer shell wrapping an outer surface of the inner shell and formed in a cylindrical shape with a cross section that is noncircular and defined by left and right small arc portions alternating with lower and upper large arc portions, the left and right small arc portions of the outer shell being contacted with the left and right small arc portions of the inner shell, respectively; and end plates secured to the outer shell to close openings of the shell main body; wherein the inner shell and the outer shell are formed by double-rolling a flat plate so the flat plate has an inner-side end portion and an outer-side end portion which are overlapped with a sandwiched portion of the flat plate located therebetween at one of the small arc portions in a circumstantial direction of the shell main body to form a shell overlap portion where the inner-side end portion and the outer end portion contact with the sandwiched portion of the flat plate, wherein the other of the small arc portions of the inner shell and the other of the small arc portions of the outer shell contact with each other, and wherein the inner-side end portion, the outer-side end portion and the sandwiched portion of the shell overlap portion are spot-welded at positions at intervals in a longitudinal direction of the shell main body so that the inner-side end portion and the sandwiched portion form a first clearance therebetween for communicating a gap formed between one of the lower and upper large arc portions of the inner and outer shell and an inner space of the main shell body with each other through the first clearance and so that the outer-side end portion and the sandwiched portion form a second clearance therebetween for communicating a gap formed between the other of the lower and upper large arc portions and an outside of the main shell body with each other through the second clearance to discharge vapor in the gaps.