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

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!