One little Google search led me to this. Plus many more, this was just the first.

What is the sight glass actually on as I cannot tell from the picture you posted?

geoffrey, the instructions to light said not to light it if the sight glass is broken. why is that? will it hurt efficiency at all if the glass isn't there?

I guess if there were a large gas build up - the flame might come out the opening.

It's a $10 part. I think it should just clip or slide on. If I can get it on then great - if not I'll probably just forget it and light the water heater anyway.

That's a water heater! I couldn't tell from your photo. I've never seen one like that on a water heater, but then I don't have a recent WH model.

If you're worried about it, you could fasten a piece of sheet metal over the hole, with small sheet metal screws or pop rivets. That said -- hmmm, why do you suppose it broke?

"hmmm, why do you suppose it broke??

I'd say whatever tore hell out of that metal retainer had something to do with breaking the glass.

I just bought the $10 part and clipped it on. Water heater lit just fine. IDK who the moron was who decided to tear the old one to shreds.