[Eljaro]

I usually repair the Smiths temp gauges myself.
Usually they fail because the gas inside has escaped through a crack in the capillary tube or some old weld at the bulb or gauge end.

Here is how I do it:

You need Ether, not more than 50cc, some brass or copper round stock about 1/4" thick, a soldering iron with at least 100W and a hair dryer or heat gun and icy water.

1.Cut the capillary tube with a fine saw closer to the gauge ( 2 feet or so). Make sure you remove all burrs and that the cut is clean and the capillary not obstructed.
2.Then you make yourself a thin copper or brass tube about a 1"long with the interior diameter the same as the exterior diameter of the capillary tube.
3.Solder this tube about 1/2" onto the capillary, making sure no solder plugs the capillary hole. Heat just enough so you see the edge of the sleeve filled with solder.
4.Get a container with water and lots of ice big enough to contain the bulb and part of the coiled up capillary tube. Wait until the water reaches the lowest temp possible.
5. You need the Ether (here I get it at Pharmacy) to fill the bulb. Keep it in the freezer until ready to use.

6. Now you heat up the bulb to aprox. 70ºC with a hair dryer or a heat gun.
7. Submerge the capillary end coming from the bulb into the bottle full with Ether.
8. Immediately after that stick the bulb into the icy water container while keeping the end of the capillary tube submerged into the Ether.
9. The cooling down bulb will draw the Ether into the tube and fill itself.
10. When the bulb has reached the low water temp, usually after less than a minute the bulb should have filled itself enough (depending on the bulb size it may need only 3 to 6 cc.
11. Keeping the bulb in the icy water you now pull the tube from the Ether and join both capillary tubes, pushing the tube coming from the gauge into the sleeve you made, bottoming up to the other end of the capillary and solder the tube to the sleeve. Do it fast before the bulb starts warming up and the Ether starts expanding and shooting out of the welding.
It is easier if you pre solder the capillary from the gauge starting about 1/2" from the end (don´t presolder the first 1/2" of the tip,or it will not fit into the sleeve you made.)to make the solder flow faster once you apply the heat with the soldering iron to join both tubes.
Apply heat with the the soldering iron with some solder just to see the edge of the sleeve covered with tin.

That is it.

You can now test the temp gauge sticking the bulb into boiling water and see if the gauge reaches the right temp. Usually 95º-98ºC depending on your altitude.