When you need to change hose diameters from 4" to 2-1/2", this fitting offers a nicely tapered transition.
The outside diameters of the ends are 4" and 2-1/2". Just push the hoses on and clamp in place. There is one special feature of this fitting that most are unaware of. The inside diameter of the small end is 2-1/4". The popular 2-1/4" shop vacuum hose fits in the end perfectly allowing you to connect a shop vacuum to a machine that has a 4" outlet. This usually is not the best situation because those machines come with a 4" outlet intended for a dust collector with a 4" inlet. But when all you have is a shop vacuum, you've got to do what you've go to do and this fitting will allow it.