These reducers allow fitting of a 2.5" dust collection hose to a 4" hose. Overall length is 3.5". Has a flange to allow easy assemble/disassemble.One end has 2.5" OD and 2.3" ID opening and the other has 3.940"OD and 3.700" ID opening. These ABS plastic reducers are very economically priced to allow you to outfit your dust collection system for maximum performance. These fittings are made from 1/8" thick ABS abrasion resistant plastic and are designed to withstand the abrasive nature of wood chips and saw dust. Due to the potential for built up of static electricity, it is recommended that all dust collection systems with plastic fittings be properly grounded.