What does MJ mean?

MJ stands for Mechanical Joint. It is a type of compression joint designed for underground connection for pipe, valves and fittings. Mechanical joints allow for some deflection of the joint. This accommodates some misalignment of the two elements connected by the mechanical joint due to earth settling, or other installation issues while maintaining a seal under pressure.

Installation requires no mechanical skill. The components are comprised of a mechanical joint bell on the pipe, valve or fitting, and a mechanical joint gland, gasket and set of tee-head bolts and nuts which are supplied loose. The spigot end of a pipe receives the gland and gasket and is then inserted into the MJ bell of the connecting pipe, valve or fitting. The tee bolts are inserted through the bell and the gland. When the bolts are tightened, the gland and bell are drawn together which squeezes the 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.

While MJ pipe has fallen out of favor with the introduction of push-on ductile iron pipe, MJ valves and fittings are still the choice of most installers. MJ valves and fittings work equally well with push-on ductile iron pipe, as well as C900 PVC pipe, and IPS PVC  pipe ( the latter requiring transition gaskets).