There are two sizes of gaskets required for the 3+1/4inch Pre-WW1 Davison sight glass. A 31mm OD gasket to fit in the top cap and a 29mm OD gasket to fit on the base of the glass.
One Top gasket and one Base gasket should be installed.
Douglas Flat Tank- Oil sight glass and gasket kit.
The kit comprises a 39mm diameter ,2mm thick, hardened sight glass plus a top and a base gasket. In addition, there is also a fine metal shim in the kit which is placed below the base gasket
Part Nr 66-462 Triumph oil sight glass and gasket kit
The engine mounted sight glass and gaskets for the late 1920s early 1930s Triumph. Models TT, ST, W, WS, N & NSD. This sight glass is best suited to the type of sight glass holder with the Screw-in type of retaining ring for the glass and gaskets and not the Push-fit type of retaining ring