A sight glass is used to visually inspect the flow of a pipe system.

These come in two Sizes 1.5 inch & 2 inch 

The 1.5 inch sight glass have a plain solvent sockets at either end - Will fit 1.5 inch Standard imperial grey pressure pipe
The 2 inch sight glass have a plain solvent sockets at either end - Will fit 2 inch Standard imperial grey pressure pipe

Will fit 1.5 inch & 2 inch Standard imperial grey pressure pipe, or use 1.5 inch to 40mm or 2 inch to 50mm reducing sleeves to get down to 40mm and 50mm  Black Solvent Waste Pipe.