In plumbing systems, what component ensures the joint is tight and prevents leaks in cast iron pipes?

The mechanical joint is specifically designed to create a tight seal between cast iron pipes, preventing leaks. This type of joint typically involves a flexible gasket that compresses as the joint is secured, ensuring a robust connection. Mechanical joints are favored in plumbing systems for their ability to accommodate slight misalignments and their effectiveness in maintaining a leak-free seal, even under varying conditions of temperature and pressure.

The other options do not provide the same level of effectiveness for cast iron pipes. Flanged joints are often used but depend on the bolting of flanges, which may not be feasible for all configurations. Screw joints rely on threading, which is not commonly applied to cast iron pipes due to potential weakening and difficulty in achieving a reliable seal. Solder joints are primarily used with copper pipes and involve melting a filler material, making them unsuitable for use with cast iron. Thus, the mechanical joint is the best choice for ensuring tightness and preventing leaks in this context.