The cap tube gauges contain a gas similar to Freon that expands when it's heated by the coolant, and the pressure moves a diaphragm connected to the gauge needle. While it might be possible to cool the gas in the gauge enough to solder the capillary tube to the old gauge without losing the gas (or liquid), it would be difficult, if not impossible to get exactly the right volume/pressure to make the gauge accurate. I'd leave the job to the professional gauge rebuilders.
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!
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