BMW Motorcycle K100RT front and rear turn signal bulbs utilize a P21W size. The replacement chart covers 1985 to 1988 production years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The P21W is the sole bulb type specified across all turn signal positions on the BMW Motorcycle K100RT, covering model years 1985-1988.
P21W is an ECE-standardized single-filament bulb rated at 12 volts and 21 watts, designed for signalling applications such as turn indicators.
The P21W uses a BA15s bayonet base, which is a single-contact bayonet fitting with a 15mm collar width.
The BA15s base is distinct from the BA15d double-contact variant; substituting one for the other is not possible due to differing contact configurations.
Both the front and rear turn signal positions on the K100RT share the same P21W specification, meaning a single bulb type serves all four indicator housings.
ECE standardization means the P21W is produced to a regulated performance specification recognized across European and many worldwide markets, ensuring consistent output characteristics across manufacturers.
At 21 watts, the P21W draws approximately 1.75 amperes on a 12-volt circuit, a figure relevant to assessing load compatibility with the motorcycle's turn signal flasher relay.
Substituting a lower-wattage bulb, such as an LED replacement not matched to the original load, may cause the flasher relay on the K100RT to operate at an incorrect flash rate without a load-compensating adapter.