An aftertreatment system and method treats exhaust gasses produced by an internal combustion process or a similar process. The aftertreatment system includes an aftertreatment module having a sleeve extending between a first end and a second end. One or more aftertreatment bricks are axially inserted into a sleeve opening disposed in the first end of the sleeve. To prevent the aftertreatment bricks from unintentionally exiting the sleeve, a captive ring is disposed around the first end of the sleeve as a loose fitting collar. The captive ring is restrained on the sleeve by a circumferential bead protruding about the first end. The captive ring can couple with a retention ring placed adjacent the first end. When coupled to the captive ring, a portion of the retention ring extends partially across and blocks the sleeve opening, preventing the aftertreatment brick from exiting the sleeve.