Bruce, took the Mamod Centurion sight glass off before to clean behind it as its getting hard to see the water level. Discovered its only made out of plastic/perspex.....that's a bit cheap on Mamod's part?
Its totally discoloured and there's no way of making it clear again.
The thought crossed my mind of cutting a piece of glass that I have which is the same thickness, the sight glass is held in with a brass holder and two screws, there's only a 'keyhole' of glass visible.