One of the handiest reducers we've seen. The small end will accept 63mm (2 1/2in) hose on the outside while the internal diameter will suit most 57mm (2 1/4in) hose end fittings like Shop Vacs etc. The big end is a tapered and will fit over most 4in dust outlets found on machines etc. Because its tapered it will push fit over the outlet, making it quick and easy to move quickly from one machine to another by simply pushing adaptor over port. Rather than being rigid plastic, it is rugged resin construction that flexes rather than cracking - great for being moved around regularly.