Lighting

Two major changes occurred in 6 volt General Motors sealed beam bulbs since they were first installed on vehicle assembly lines in 1940. The perfectionist restoring his truck to exact original specifications must have the correct sealed beams for his year. In very competitive judging, it’s these details that can make a difference. No doubt, replacement bulbs from a GM dealership with slow sales could sometimes be placed on newer vehicles a few years after the units were discontinued, however this article is based on bulbs you would have bought new from the factory during that particular year.

1940 through about 1946 – These sealed beams had a double filament small bulb built inside. The large glass reflector in the back was sealed from the elements.  It stayed bright until the bulb burned out.  It was not like earlier open reflectors that could tarnish with age.  The assembly comes with a metal black back attached. If the outer glass gets a rock hole, the light continues to work well. The filament is still encased in the smaller argon gas filled glass bulb (see photo).

1940 through about 1946 (above)

About 1946 through 1955 (below)

The first sealed beam bulb, as we know it, was introduced after the end of World War II. Between the reflector and the outer glass covering is the open unprotected filament (no small internal bulb). The total interior is filled with argon gas to protect the filament from air which causes instant burn out. When a rock places a small hole in the glass the filament is ruined.

The big visible difference in this first real sealed beam bulb and today is there was no aiming bumps on the outside as in later years. The dealer aiming equipment had not been invented.

1955 and newer…(above)

Beginning in 1955 the 6 and 12 volt sealed beams have three glass aiming bumps molded in the edges of the lens. The bumps were needed by new light aiming equipment provided to GM dealerships. All of these GM bulbs with aiming bumps have the letters T-3 molded in the center of the glass lens. Most will also have the word Guide at the top of the lens.

Note: These modern bumps will interfere with properly attaching the chrome bezels on a 1940 headlight bucket as well as a 1937-1939 bulb light that has been converted to sealed beams. The bezels were not designed for the bulb still 15 years in the future. The 1940 GM vehicle owners will have a long hunt to find sealed beam bulbs without the three bumps.