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Karl,You can fit sight glass to a 3" boiler without any trouble, Here is a pic of a 4" Horizontal Scotch return flue boiler with the top sight glass fitting coming from a Banjo fitting on the top of the boiler set at an angle to clear the chimney with an extension to take the top sight glass fitting and the bottom fitting into the end plate.If you wish I can post drawings from Model Engineer to make your own fittings.George.