An improved apparatus for trimming an irregular open end of a container body is disclosed. The improved trimmer cartridge includes a sealed gear housing, for retention of interior lubricant and to reduce contamination. An adjustment mechanism, accessible from the exterior of the housing, is provided to adjust a gap between two blades of the trimmer cartridge. The adjustment mechanism provides improved control over the gap width, which is between 8-25% of the can metal thickness. Additionally, a tang that is removably connected to a shaft for a first blade provides rotational power for the trimmer. The tang is replaceable without requiring the housing to be opened or requiring replacement of the blade shaft. Another blade shaft is provided for a second blade, with the two blade shafts having substantially equal mass. The trimmer also includes spur gears, which do not have the thrust load associated with prior art helical gears.