Fireproof replacement furnace burner sight glass measures 1-11/16" in diameter. This is our medium size. We also sell two other sizes. Small size is part # SG-510 and our large size is part # SG-512.
Also available separately for this medium sight glass is a slip-ring nut (SN-501) and a high-heat gasket (G-181).
Note. If you are looking for replacement glass for the Coleman 7660-3661 observation cap, this glass and the G-181 gasket will fit. However, the SN-511 slip-ring nut will not thread correctly onto the Coleman furnace. For a replacement, order our shop-made 7660-3661 replacement.