Helicopter Dual Tachometer Kollsman 981BX-8-023 H-21 Piasecki
Here is a dual Tachometer for use in military helicopters, indicating RPM for the rotor on the inner scale and rotor for the outer scale with a ratio of 9.7:1. The pointers will be aligned once the rotors are fully engaged. It was used in the H-21 Shawnee/Workhorse Piasecki Helicopter. There are some markings on the glass denoting the operating limits and orientation of the glass (the white mark, just in case it rotated). Measures 3.25 inches diameter by 5 inches deep.