The Nardi design steering wheel is delivered without horn button and hub, you will have to order them additionally. Also check if you can reuse the horn slip ring from your old handlebars, if not it is mentioned on the 'related' tab (just like the steering wheel and hub).
This aluminum horn button is not a replacement for the button of an original Nardi steering wheel.
If you want to use the horn button for another steering wheel or steering hub, the installation diameter is listed.
Installation Instruction
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