Since this is part of the conversion from an R18 to a Roctane headlight, but at the same time an independent and very difficult part, I have written a separate guide for it in the repair Wiki.
https://www.r1800wiki.org/en/2024/07/11/tachometer-zerlegen/
https://www.r1800wiki.org/en/2024/07/11/tachometer-zerlegen/