A new design without threadsAdd a 16mm glass marble between the two parts, assemble with 6 M3 screws and nuts. You may also add a spring if you cannot install the check valve in such way that the gravity push the marble in its location. You may also add a paper or rubber joint between the parts to avoid leaks. This joint can be cut in a bicycle inner tube for example. It is designed for 21 mm inner diameter of household dishwasher or clothe-washer water outlet.