MJ stands for Mechanical Joint and is a compression type of fitting. A mechanical joint is a time-saving, easily installed joint that insures long-term, trouble-free performance. Uncomplicated, this joint comprises a gland, gasket and bolts. It requires no mechanical skill to install, is unusually easy and fast to assemble.
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.