The flash tubes are what carry the flame between the burners. They are an essential part of your grill as the primary burner is the only burner that is lit by the ignitor. Once the primary burner is lit, special holes in the flash tubes allow the flame to jump between each of the burners. For the LM210-28, you will receive two flash tubes, and for the LM210-40, three flash tubes. Some older versions of the units came with a “flash bar” which spanned across all the burners; these flash tubes replace all previous revisions.
On newer fireboxes, they fit into the screw-holes provided in the firebox. Just use the provided screws to slide down and lock in the flash tubes to the rear of the firebox.
If you have a older firebox you will need to remove the firebox and screw them in from the outside. This means you need to remove the firebox completely from the grill to access the rear. Sometimes the heads of the screws will rust off, leaving little trace where the holes are. If you can not easily identify the hole from the inside, you should look on the outside of the firebox and you should be able to see the remnants of the screw sticking though. The holes are located about half way between each of the burners. You will need to drill a new hole into the firebox to attach the flash tubes. The alternative is you can purchase a renewal kit or a replacement firebox which come pre-fabricated with the screw-holes.