An assembly for pre-stressing a canister within a magnetically coupled device having a housing body, a housing head and a canister connectable to the housing body, a pin and pin plate combination disposed between the housing head and the canister, and wherein the housing body, housing head, pin and pin plate combination and canister are adapted to cause the canister to stretch in a longitudinal direction when the housing head and canister are connected in sealing engagement with the housing body. A method of stretching a magnetic coupling canister also is disclosed.