That "bare" wire isn't a wire- - - -it's a copper capillary tube attached to a bulb that's bolted into the cylinder head. The bulb and the tube are filled with a volatile liquid- - - -usually ether. Eventually, the tube gets damaged or springs a leak, and the needle stops moving. That type of gauge is almost impossible for a backyard tinkerer to fix. I have had some success buying a cheap mechanical gauge and swapping the liquid-filled bulb to an original stovebolt gauge head, but it requires an extremely cold liquid to condense the pressure medium like dry ice and alcohol, or maybe liquid nitrogen.
Jerry
