I repaired my TR3 temp gauge and it was relatively straightforward, assuming you can operate a soldering iron. The innards of the gauge itself are fairly simple - a metal diaphragm that expands under pressure (from the heated ether) and pushes a rod that moves the needle. Our gauge didn't work because we were sort of forced to cut the capillary line that runs between the gauge and the ether bulb that fits in the water pump, so all our ether went bye bye. Read the instructional link in the post from martx-5, but the general idea is to buy a cheap $20 capillary tube new gauge from the parts store, chill the new bulb waaaay down by immersing it in a mixture of water, ice and salt, then splice the existing tube from the old gauge to the new tube from the new gauge (important - remember to put on the proper securing nut to fit the TR3 water pump before splicing!!). The splice is made with a 1 inch section of 1/8 inch or so brass tubing. You tin the clean-cut ends of the capillary tubes (only the outsides - don't stop up the tubes!). Also it helps to insure that the tube ends were not crimped shut when you cut them - if so, open them up with the sharp end of a safety pin or something similar. Once tinned, the ends go in the section of brass tube and you solder the ends into the tube, ensuring *very* clean solder joints with no cracks or open areas. Once done, you can remove the new bulb from the freezing water and insert it into hot water (or just hold it under a very hot faucet) - you should see some movement on the needle as the bulb heats up. If you have any kind of leak around your solder joint you will be able to smell it quite easily. Dipping it in boiling water should send the bulb towards 212 and when back under cool running water it should go back down. So long as it swings most of the way across the dial, you are probably good to go and can adjust the needle to be reasonably accurate. Once repaired, I also used a model brush to repaint the needle using white model lacquer to give it a cleaner look.