An adjustment mechanism for a round and irregular seamer useful in the manufacture of tins is disclosed. The seamer has a rotating usually circular plate sandwiched between stationary cam plates above the plate and a seaming chuck below the plate. A seaming assembly is rotationally mounted to the rotating plate proximate its periphery and is provided above the plate with a cam follower which interacts with the cam plate and beneath the plate with a seaming arm and head which moves towards and away from the seaming chuck depending on the interaction of the cam follower with the cam plate.