The GMC 370 uses a 63 bulb size for both front and rear side marker applications. The replacement chart covers 1951 through 1955 production years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The GMC 370 uses the 63 bulb in both the front and rear side marker positions across the 1951-1955 model years.
The 63 bulb follows the ANSI standard, which is the American National Standards Institute designation system used in the United States and Canada to classify automotive bulbs by a numeric code.
As a single-filament bulb, the 63 serves only one lighting function per socket, as opposed to dual-filament bulbs that can serve two functions such as running and braking from a single unit.
The 63 bulb operates on a BA9s base, which is a miniature bayonet cap with a 9mm collar width. This base type is common in low-wattage indicator and marker applications.
The 63 bulb is rated at 12 volts, consistent with the standard electrical system voltage found in conventional automotive applications.
At 0.63 watts, the 63 bulb draws very low power, which is characteristic of side marker lamps that function as passive visibility indicators rather than primary illumination sources.
The shared bulb type across both the front and rear side marker positions on the GMC 370 means a single replacement bulb serves all four side marker sockets on the vehicle.