Big Dog Mastiff high beam and low beam headlight bulbs both utilize a 9003 bulb size. The replacement chart covers 2010 through 2011 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 9003 is the sole bulb type specified for the Big Dog Mastiff across both the 2010-2011 model years, covering both high beam and low beam headlight positions.
The 9003 is a dual-filament halogen bulb, meaning a single bulb unit contains two separate filaments: one rated at 60W for high beam output and one rated at 55W for low beam output.
Because the 9003 serves both the high beam and low beam positions on the Mastiff, both functions are handled by the same physical bulb rather than by separate dedicated bulbs for each function.
The 9003 uses a P43t base, which is a bayonet-style mount with a 43mm flange. Bulbs with a different base type are not physically compatible with this socket, regardless of voltage or wattage ratings.
The 9003 operates on a 12V electrical system, which is standard for most motorcycle and automotive applications in this category.
The 9003 is defined under ANSI standards and is recognized in both the United States and Canada. It is functionally equivalent to the H4 bulb used in international markets, though fitment interchangeability depends on the specific socket design of the vehicle.