Q."Why are there pipe sizes listed for these couplings, but then the corresponding outside diameter of the pipe they fit is different?" A. No-hub couplings were designed primarily to fit cast-iron pipe, but can also fit many types of plastic and steel pipe; the size of such pipe is determined by the "nominal" (or approximate) inside diameter of the pipe. We list the approximate outside diameters, too, so you'll be able to determine whether or not they will fit your pipe.
Q."Can I use a No-Hub coupling in place of my flexible rubber coupling?" A. As long as both pipe sizes match up to the shielded no-hub coupling sizes then yes, you can. However, you should not replace a no-hub coupling with a flexible rubber coupling if it is an above ground installation or repair. Flexible rubber couplings are for use underground, but no-hub couplings can be used above or below ground.
Q."Why does the no-hub coupling have a shield when it seems the flexible rubber coupling is thicker and can do the same job?" A. The shield was designed for the no-hub coupling to adjust to variations in diameters of the pipes to be joined. When the bands are tightened the corrugations line up which causes pressure both parallel and crossways against the gasket which in turn tightens against the pipe providing a reliable positive seal. The shield keeps the pipe from shifting and possibly cutting or breaking through the gasket and the gasket creates the seal and prevents the pipe from pulling out.