BMW Motorcycle K1100LT front and rear turn signal bulbs use a P21W size. The replacement chart covers 1993 to 1999 model 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 BMW Motorcycle K1100LT uses the P21W bulb type for both front and rear turn signals across all model years from 1993-1999.
The P21W is an ECE-standard bulb, meaning it conforms to the Economic Commission for Europe regulatory framework, which governs lighting equipment used in European and worldwide automotive applications.
P21W bulbs operate on a 12-volt electrical system and draw 21 watts, which is the standard wattage for single-filament turn signal applications in this class of vehicle.
The P21W uses a BA15s bayonet base, where "BA" denotes a bayonet cap fitting, "15" refers to the 15mm base width, and the lowercase "s" indicates a single contact at the base, distinguishing it from dual-contact variants.
Because the P21W is single-filament, it serves a dedicated signalling function and is not wired to perform a secondary role such as braking or running light operation.
The uniform use of the P21W across both the front and rear positions means the K1100LT requires only one bulb type for all four turn signal housings, front and rear.
Substituting a dual-contact bayonet bulb, such as the BAY15d-based P21/5W, would be mechanically incompatible with the BA15s socket on the K1100LT due to the differing pin configuration.
Replacing the P21W with a bulb of a different wattage rating risks triggering a hyper-flash condition, as the motorcycle's turn signal relay is calibrated to the expected 21-watt load.