Sight windows have a glass type coating over the polycarbonate that is very thin. This makes the window resistant to chemicals, solvants, & petrolium oils. The thin glass coating may crack over time because the coating does not flex or expand with the heat the same as the polycarbonate window does. Sight windows are very easy to change and is reconmended to have as a spareincase a crack develops in thecoating. Filter window reatainers are engraved with apart number located near the center on the 6" filters or bythe port openings on 4" filters that can be viewed through the sight window, thispart number will reference thethickness of the replacement window