OEM tachometers were designed so that the hourmeter would activate only when a predesignated RPM was reached. Most tachometers have this RPM listed on the face of the tachometer. The hours WILL NOT move until the RPM range has been reached.
For example, a Ford 8N tachometer RPM is preset to activate the hour meter at 1580 RPM. A tachometer for a Ford 861 5-speed is preset at 1515 RPM. The tachometer hourmeter will only activate at or above this RPM.
I hope this helps with your restoration project, good luck!