Glass that fits

Chamber or flaretop: reading your burner before you order a chimney, so the new glass seats instead of rocks.

Two shapes, no swapping

Chamber lamps seat the 411B glass: straight at the base, made for a chamber burner. Lantern style burners take the 417B flaretop. The shapes do not interchange; a wrong glass rocks on the collar instead of seating.

Read the burner first

The model number on the burner collar settles the question before measuring does. No number? Look at the seat: a flat ring wants the chamber glass, a flared cup wants the flaretop.

While the glass is off

A chimney change is the cheapest moment to look at the rest of the lamp. Check the wick for hard char and the burner threads for wobble; both parts are on the same shelf as the glass.