Adapter connects 2" ID SCH40 PVC to Central Vacuum PVC 2" OD. It is never recommended that SCH40 is used in a central vacuum system. It is not created for the smooth travel of air and debris but rather water. Clogs are bound to happen. But if the system is nearly roughed in and the wall mounts will not connect to SCH40 use one of these adapters per inlet. Come off the mount with a central vac fitting, some pipe and then this adapter into the SCH40 pipe. Outer diameter of Schedule 40 is 2-3/16". Adapter inner diameter on one side is 2" and the other side 2-3/16".