The Buick GS 400 front turn signal requires a 1157NA bulb size while the rear turn signal uses a 1157 bulb size. The replacement chart covers 1968 to 1969 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 1157NA and 1157 bulbs are both ANSI-standard dual-filament bulbs operating at 12V with a 21W/5W power rating and a BAY15d bayonet base.
The dual-filament design of both bulb types allows a single bulb to serve two functions, with the higher 21W filament typically handling the turn signal or brake light function and the lower 5W filament handling the running light function.
The BAY15d bayonet base features two offset pins, which physically prevents incorrect insertion and ensures consistent filament alignment within the housing.
The sole distinction between the 1157NA and the 1157 is the natural amber glass envelope of the 1157NA, which produces an amber light output without relying on a colored lens.
For the Buick GS 400, the front turn signal position uses the 1157NA across both the 1968-1969 model years, while the rear turn signal position uses the standard 1157 across the same years.
Because the 1157NA and 1157 share the same BAY15d base and electrical ratings, they are physically interchangeable, though substituting a clear 1157 in a front housing designed for the 1157NA would alter the emitted light color if the lens itself is not amber-tinted.