Almost all of the headlights during at least the mid to late 1930’s had double filament bulbs. They were 32cp (candle power) on either the low or high beam setting. This made replacing the burned out bulb a “no brainer”. You just pushed the bulb in the receiving socket and gave it a slight turn. This locked the bulb in place.
Even if you were 180 degrees different each time the bulb was added, it made no difference. Both high and low beam filament in bulb was 32cp.
NOW ENTERS A NEW VARIABLE. An aftermarket company later introduces a new bulb with a brighter high beam! It is rated 32cp low and 50cp high. Better roads allowing faster driving needed a brighter high beam.
Because the original bulb receiving sockets remain the same, this new 32cp / 50cp bulb must be placed in the socket just right! In other words, the high beam 50cp contact of this modern light bulb must contact the high beam wire in the harness. The bulb being added can only fit one of two ways. If incorrect you will have the 50cp as the low beam. Not good!
To prevent this low-high beam problem from occurring on the assembly line or at least at the dealership, a different bulb base was introduced. In 1937 on new vehicles and continued until the sealed beam began in 1940. The different holes in the flat ring around the bulb base prevented a mistake. The bulb could only be attached in one way. Most all vehicles by then were 32cp/50cp.
1934-36 Duel Filament Bulb
1937-39 Duel Filament Bulb
The Earliest Sealed Beam Bulbs
Two major changes have occurred in General Motors sealed beam bulbs. The early version are actually not sealed beams as we buy them today. They were first installed on vehicle assembly lines in 1940. The perfectionist restoring his truck to exact original specifications must have the correct headlights for his year. In very competitive judging, it’s these details that can make a difference. No doubt, replacement bulbs from a small GM dealership 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.
From 1940 through about 1955 seal beams had a double filament small bulb built inside. The large glass reflector in the back was sealed from the elements. It stayed bright even after the inside bulb burned out. It was not like earlier open reflectors that could tarnish with age due to the silver plating. The assembly comes with a metal black back attached for support. 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 through in the photo it is hidden behind the large glass cover.
1940 through about 1955 (above)
More Modern Seal Beam Bulb
The first truly sealed beam bulbs, as are in auto part stores today, were introduced about 1955. 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.
It is suspected rural car and truck owners quickly learned to stay their distance from the vehicle ahead with these new design seal beam. A flying rock causing a small hole in the glass can total the new sealed beam instantly.
1955 and newer (above)
Beginning in 1955 both the 6 and 12 volt sealed beams have the three glass aiming bumps molded in the edges of the lens. The bumps were needed by new light aiming equipment provided to most all dealerships. These early second series GM bulbs with aiming bumps have the letters T-3 molded in the center of the glass lens. Most sold by the GM dealerships will also have the word Guide at the top of the lens.
Note: These modern bumps will interfere with properly attaching the chrome factory bezel on a 1940 Chevy/GMC headlight bucket as well if 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.
It is interesting to note that the small two filament bulbs before 1940 had only a pair of contacts on their base. The bulbs were grounded by the metal reflector and the through the light housing.
A three wire plug was pressed to the seal beam in 1940 and newer. In this way the lighting had a ground wire which would carry the current to a solid metal part of the chassis. This gave less chance of a dimming light from rust or related corrosion at connection points.
