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MJ stands for Mechanical Joint and is a compression type of fitting. When the bolts are tightened, two metal pieces are drawn together, which squeezes a rubber gasket that is between them. This compresses the gasket forcing it to press tightly against the pipe and make a seal. An MJ seal is rated for 350 psi for 2″ to 24″ fittings, and to 250 psi for 30″ to 36″ fittings.
