bought a used tachometer (rev counter) from a well known auction site
Was it certified for the installation? Yeah, I know it was a permit aircraft, but this does not exempt it from the need to have the
correct parts installed!
Different tachometers have been made to account for the differing output speeds of the engine tach drive - it is not always 1:1.
I hope that "hour building" would be of lesser interest to a pilot than gaining piloting experience.